The longevity field is full of jargon — NAD+, mTOR, AMPK, epigenetic clocks, autophagy, senolytics. If you're new to the space, it can feel like learning a new language. This glossary defines the 50 most important terms you'll encounter, organized alphabetically with quick-jump links at the top.
This is a reference page, not an article. Use the A-Z navigation below to jump to a specific letter, or browse the full list. Each term includes a clear definition plus, where relevant, links to deeper guides on our site. If you're looking for answers to specific questions rather than definitions, try our Longevity FAQ instead.
For terminology specific to biological age testing, see also our Biological Age Explained guide. For terminology around supplements, see the individual supplement guides linked throughout this glossary.
Letter A
AMPK
AMP-activated protein kinase. A cellular energy sensor that activates when cellular ATP (energy) levels drop — for example, during exercise, fasting, or caloric restriction. When AMPK turns on, it triggers processes that promote cellular repair, mitochondrial biogenesis, and fat oxidation. AMPK and mTOR act as a seesaw: when one is active, the other is suppressed. Exercise, metformin, and berberine all activate AMPK.
Apigenin
A naturally occurring flavonoid found in parsley, celery, chamomile, and other plants. Apigenin has been studied for its potential to activate sirtuins and suppress CD38 (an enzyme that degrades NAD+), making it a popular add-on to NMN or NR supplementation. Human clinical data is still early; see our Best Apigenin Supplements guide.
Autophagy
From the Greek for 'self-eating.' The process by which cells recycle damaged or dysfunctional components — misfolded proteins, worn-out mitochondria, and other cellular debris. Autophagy declines with age, and reduced autophagy is implicated in many age-related diseases including Alzheimer's, Parkinson's, and cancer. Fasting, exercise, spermidine, and rapamycin all activate autophagy.
Letter B
Beta-hydroxybutyrate (BHB)
The primary ketone body produced by the liver during fasting, caloric restriction, or a ketogenic diet. BHB serves as an alternative fuel source to glucose and also acts as a signaling molecule, influencing gene expression and inflammation. BHB levels rise during prolonged fasting and are associated with many of the metabolic benefits of ketosis.
Biological age
An estimate of how old your cells and tissues behave, as opposed to your chronological age (years since birth). Biological age is typically measured using DNA methylation patterns (epigenetic clocks) and is a stronger predictor of age-related disease risk than chronological age. See our Biological Age Explained guide.
Blue Zones
Regions identified by National Geographic researcher Dan Buettner where people live measurably longer, healthier lives: Sardinia (Italy), Okinawa (Japan), Nicoya (Costa Rica), Icaria (Greece), and Loma Linda (California). Common lifestyle features include daily low-intensity movement, plant-forward diets, strong social connection, and a sense of purpose.
Letter C
Caloric restriction
A dietary regimen that reduces calorie intake (typically by 20-40%) without causing malnutrition. Caloric restriction extends lifespan in nearly every species studied, from yeast to mice. Human data is more limited, but caloric restriction appears to improve markers of metabolic health and reduce age-related disease risk. The practical downside is muscle loss, hormonal disruption, and difficulty sustaining the regimen long-term.
CGM (Continuous Glucose Monitor)
A small wearable sensor (typically worn on the upper arm) that measures interstitial glucose levels continuously, usually every 1-5 minutes, for 10-14 days per sensor. Originally developed for diabetes management, CGMs are increasingly used by non-diabetics to understand their metabolic responses to foods, exercise, stress, and sleep. See our Best CGMs for Non-Diabetics guide.
CoQ10 (Coenzyme Q10)
A compound that plays a critical role in mitochondrial energy production (it's part of the electron transport chain) and acts as an antioxidant. CoQ10 levels decline with age, and statin use further depletes CoQ10. Supplementation may be especially valuable for statin users and older adults. See our related supplement guides for product recommendations.
Creatine
A nitrogenous organic acid that helps recycle ATP (cellular energy) in muscle and brain tissue. Creatine monohydrate is one of the most studied and effective supplements in existence — it improves high-intensity exercise performance, increases lean mass, and has emerging evidence for cognitive benefits. 5g per day is the standard dose. See our Best Creatine Supplements guide.
Letter D
DEXA scan
Dual-energy X-ray absorptiometry. The gold-standard method for measuring body composition — bone mineral density, lean mass, and fat mass — with high accuracy. A DEXA scan takes 10-15 minutes, exposes you to a tiny amount of radiation, and is invaluable for tracking muscle mass (preventing sarcopenia) and bone density (preventing osteoporosis) over time.
DNA methylation
A chemical modification to DNA (the addition of a methyl group to cytosine bases, typically at CpG sites) that changes gene expression without changing the underlying DNA sequence. DNA methylation patterns change predictably with age, and these changes form the basis of epigenetic clocks (biological age tests). DNA methylation is one of the most well-studied 'hallmarks of aging.'
Letter E
ECG (Electrocardiogram)
A recording of the electrical activity of the heart, used to detect arrhythmias, atrial fibrillation, and other cardiac conditions. Modern wearables like the Apple Watch Ultra 2, Withings ScanWatch, and Fitbit Sense 2 can capture single-lead ECGs that may detect atrial fibrillation. These are not diagnostic on their own but can prompt clinical follow-up.
EDA (Electrodermal Activity)
A measure of skin conductance that reflects sympathetic nervous system activity (the 'fight or flight' branch). EDA is used as a proxy for stress levels. The Fitbit Sense 2 has a dedicated EDA sensor and uses EDA data in its stress management score. EDA is also used in polygraph (lie detector) testing.
Epigenetic clock
A statistical model that estimates biological age from DNA methylation patterns at specific CpG sites. The first widely-used clock was the Horvath Clock (2013). Newer clocks — PhenoAge (2018), GrimAge (2019), DunedinPACE (2022) — are trained on clinical outcomes (mortality, disease) rather than chronological age and are stronger predictors of age-related risk. See our Best Biological Age Tests guide.
Epigenetics
The study of heritable changes in gene expression that do not involve changes to the underlying DNA sequence. DNA methylation and histone modification are the best-known epigenetic mechanisms. Aging involves predictable epigenetic changes, and the reversibility of these changes is one of the most exciting frontiers in longevity research.
Letter F
Fasting (intermittent)
A dietary pattern that cycles between periods of eating and not eating. Common protocols include 16:8 (16-hour fast, 8-hour eating window), 18:6, 5:2 (5 days eating normally, 2 days restricted), and prolonged fasts (3-5 days). Fasting activates AMPK, autophagy, and other longevity-associated pathways. See our Intermittent Fasting Protocols guide.
Fisetin
A flavonoid found in strawberries, apples, and other fruits. Fisetin has been studied as a senolytic — a compound that selectively kills senescent (zombie) cells. Most evidence to date is in cell and animal models; human clinical trials are ongoing. Fisetin is sometimes stacked with quercetin and other senolytics in periodic 'senolytic pulse' protocols.
Frailty
A clinical syndrome characterized by decreased physiological reserve and increased vulnerability to stressors. Frailty involves muscle loss, weakness, low activity, slow walking speed, and unintentional weight loss. It is strongly associated with mortality and disability. Sarcopenia (muscle loss) is a major driver of frailty, which is why strength training in older adults is so important for longevity.
Letter G
Glycation (AGEs)
The non-enzymatic binding of sugar molecules to proteins or lipids, forming advanced glycation end-products (AGEs). AGEs accumulate with age and damage tissues — they crosslink collagen (causing stiff arteries and wrinkled skin), promote inflammation, and contribute to diabetic complications. High blood sugar accelerates glycation, which is one reason glucose control matters for longevity.
Letter H
Healthspan
The portion of life spent in good health — free from chronic disease, disability, and cognitive decline. The goal of longevity science is to extend healthspan faster than lifespan, so that you stay vital and capable for most of your life. Modern medicine has extended lifespan more than healthspan, leaving most people with 10-15 years of poor health at the end of life.
HRV (Heart Rate Variability)
The variation in time between successive heartbeats, measured in milliseconds. Higher HRV generally indicates better cardiovascular fitness, lower stress, and better recovery; lower HRV can signal illness, overtraining, or chronic stress. HRV is most useful as a personal trend over time, not as an absolute number to compare to others. Tracked by Oura, Whoop, Apple Watch, and Garmin.
Letter I
Inflammaging
Chronic, low-grade inflammation that develops with age and contributes to many age-related diseases including cardiovascular disease, cancer, neurodegeneration, and frailty. Inflammaging is driven by cellular senescence, gut permeability changes, visceral fat accumulation, and declining immune function. Reducing inflammaging is a major target of longevity interventions including exercise, sleep, omega-3 supplementation, and stress reduction.
Letter L
Lifespan
The total number of years a person lives, measured from birth to death. Distinguished from healthspan (years lived in good health) and longevity (the science of extending both). Maximum human lifespan (the longest any human has lived) is currently 122 years (Jeanne Calment). Average life expectancy varies by country and socioeconomic status.
Liposomal delivery
A drug/supplement delivery technology that wraps the active compound in a lipid bilayer (a 'liposome'), improving absorption and bioavailability. Liposomal delivery can dramatically increase blood levels of poorly-absorbed compounds like NMN, glutathione, and curcumin. Liposomal supplements cost more than standard capsules but may deliver more active compound per dollar.
Letter M
Magnesium
An essential mineral involved in over 300 enzymatic reactions, including ATP metabolism, muscle and nerve function, blood glucose control, and blood pressure regulation. Most adults don't get enough magnesium from diet alone. Common forms: glycinate (good for sleep, well-absorbed), threonate (crosses blood-brain barrier, may support cognitive function), citrate (well-absorbed, can cause loose stools), oxide (poorly absorbed). See our Best Magnesium Supplements guide.
Metformin
A prescription drug used to treat Type 2 diabetes. Metformin activates AMPK and has shown signals of longevity benefit in observational data — diabetics on metformin sometimes outlive non-diabetics. The TAME (Targeting Aging with Metformin) trial is testing metformin for longevity in non-diabetics. Metformin has side effects (GI, B12 depletion, possible blunting of exercise adaptations) and should not be used without physician oversight.
Microbiome
The community of microorganisms (bacteria, fungi, viruses) that live in and on the human body — most commonly refers to the gut microbiome. The gut microbiome influences digestion, immune function, inflammation, metabolism, and even mood. Microbiome composition changes with age, diet, antibiotics, and stress. Microbiome testing is increasingly popular but still maturing clinically.
Mitochondria
The 'powerhouses' of the cell — organelles that generate ATP (cellular energy) via oxidative phosphorylation. Mitochondria have their own DNA (mtDNA) and replicate independently. Mitochondrial function declines with age, and mitochondrial dysfunction is a key hallmark of aging. Exercise (especially Zone 2 cardio) is one of the strongest stimulators of mitochondrial biogenesis and health.
Mitophagy
A specialized form of autophagy that selectively removes damaged or dysfunctional mitochondria. Mitophagy declines with age, contributing to the accumulation of dysfunctional mitochondria that produce less ATP and more reactive oxygen species. Exercise, urolithin A, spermidine, and fasting all promote mitophagy.
mTOR
Mechanistic (formerly mammalian) target of rapamycin. A nutrient-sensing cellular pathway that promotes growth when nutrients (especially amino acids) are abundant and suppresses autophagy. Chronic mTOR activation (from constant eating, high protein, low movement) is associated with accelerated aging. mTOR inhibition — through fasting, protein restriction, or rapamycin — activates autophagy and extends lifespan in animals.
Letter N
NAD+
Nicotinamide adenine dinucleotide. A coenzyme found in all living cells that's essential for energy metabolism (it carries electrons in redox reactions) and for the activity of sirtuins (longevity-associated enzymes) and PARPs (DNA repair enzymes). NAD+ levels decline with age — by 50% or more between ages 40 and 60. NAD+ precursors like NMN and NR are among the most popular longevity supplements.
NMN (Nicotinamide Mononucleotide)
A direct precursor to NAD+ that has been shown to raise NAD+ levels in humans in multiple clinical trials. NMN is one of the most popular longevity supplements, made famous by David Sinclair's research at Harvard. Typical dose is 250-500mg per day. NMN is more expensive than NR but appears to be more efficient at raising NAD+ in some tissues. See our Best NMN Supplements guide.
NR (Nicotinamide Riboside)
Another NAD+ precursor, often sold as Niagen (a patented form). NR has more published human clinical data than NMN and is generally less expensive per dose. NR is well-absorbed and reliably raises NAD+ in blood. Both NMN and NR are reasonable choices; pick based on price, form, and personal response. See our Best NR Supplements guide.
Letter O
Omega-3
A family of polyunsaturated fatty acids (the most important being EPA and DHA, found in fatty fish and algae) that are essential for brain health, cardiovascular function, and inflammation regulation. Most adults don't get enough omega-3 from diet alone. Supplementing with 1-2g of EPA+DHA per day is one of the best-supported longevity interventions. See our Best Omega-3 Supplements guide.
Letter P
PhenoAge
A second-generation epigenetic clock developed by Morgan Levine and colleagues (2018). Trained on a composite of clinical biomarkers (glucose, creatinine, CRP, white blood cell count, etc.) rather than chronological age, making it a stronger predictor of mortality and age-related disease than the original Horvath clock. Used in TruDiagnostic and Elysium Index biological age tests.
PPG (Photoplethysmography)
An optical technique used by virtually every wrist-worn wearable (Apple Watch, Fitbit, Garmin, Whoop) to measure heart rate. PPG works by shining green (and sometimes red/infrared) light into the skin and measuring the reflected light, which changes with blood flow. PPG is accurate for resting heart rate and reasonably accurate for exercise heart rate, but less reliable during high-intensity or wrist-bending activities.
Letter Q
Quercetin
A flavonoid found in onions, apples, capers, and other plants. Quercetin has been studied as a senolytic (selectively kills senescent cells), often paired with dasatinib in research settings or with fisetin in consumer supplements. Quercetin has poor bioavailability on its own; liposomal and phytosome formulations improve absorption. Human senolytic data is still early.
Letter R
Rapamycin
A prescription immunosuppressant (originally approved to prevent organ transplant rejection) that inhibits mTOR. Rapamycin consistently extends lifespan in mice, even when started late in life, making it one of the most promising pharmacological longevity interventions. Low-dose rapamycin (typically 3-6mg once weekly) is increasingly prescribed off-label by longevity clinicians. Side effects at higher doses include immune suppression, mouth ulcers, and metabolic effects.
Resveratrol
A polyphenol found in red wine, grapes, and berries that activates sirtuins (longevity-associated enzymes) in cell and animal studies. Made famous by David Sinclair's research. Human clinical trials have been mixed — resveratrol has poor bioavailability and the optimal dose for humans is unclear. We still take it (low risk, low cost, reasonable theoretical case) but view it as a lower-priority supplement than NMN, omega-3, or creatine. See our Best Resveratrol Supplements guide.
ROS (Reactive Oxygen Species)
Chemically reactive molecules containing oxygen (e.g., superoxide, hydrogen peroxide) generated as byproducts of cellular metabolism, especially in mitochondria. At low levels, ROS serve important signaling functions. At high levels, ROS cause oxidative damage to DNA, proteins, and lipids — a process called oxidative stress. ROS imbalance is implicated in many age-related diseases, but indiscriminate 'antioxidant' supplementation has shown disappointing results in clinical trials.
Letter S
Sarcopenia
Age-related loss of skeletal muscle mass and strength. Sarcopenia typically begins in the 30s-40s and accelerates after 60. It's a major driver of frailty, falls, disability, and mortality in older adults. Resistance training and adequate protein intake (especially after age 60) are the most effective interventions. DEXA scans track muscle mass over time.
Senescence (cellular)
A state in which cells stop dividing but don't die — they become 'zombie cells' that secrete inflammatory molecules (the senescence-associated secretory phenotype, or SASP). Senescent cells accumulate with age and contribute to inflammation, tissue dysfunction, and age-related disease. Senolytics (compounds that selectively kill senescent cells) like fisetin, quercetin, and dasatinib are an active area of longevity research.
Sirtuins
A family of seven proteins (SIRT1-7) in humans that act as cellular stress sensors and epigenetic regulators. Sirtuins are NAD+-dependent — they require NAD+ to function, which is one reason NAD+ decline with age matters. Sirtuins influence DNA repair, inflammation, mitochondrial function, and circadian rhythms. Activated by caloric restriction, fasting, exercise, and compounds like resveratrol.
Spermidine
A polyamine found in wheat germ, soy, mushrooms, and aged cheese that induces autophagy. Spermidine levels decline with age. Epidemiological data suggests higher dietary spermidine intake is associated with lower mortality. Spermidine supplementation is generally safe and inexpensive. See our Best Spermidine Supplements guide.
SpO2 (peripheral capillary oxygen saturation)
A measure of the percentage of hemoglobin in arterial blood that's saturated with oxygen. Healthy values are 95-100%. SpO2 is measured non-invasively via pulse oximetry (the red/infrared light clip on your finger, or the optical sensor on a wearable). Most modern wearables (Apple Watch, Fitbit, Oura, Withings) can measure SpO2, though accuracy varies and clinical-grade pulse oximetry remains the gold standard.
Letter T
Telomeres
Repetitive DNA sequences (TTAGGG repeats in humans) at the ends of chromosomes that protect them from degradation. Telomeres shorten with each cell division, and critically short telomeres trigger cellular senescence or apoptosis. Telomere length is one of the older 'hallmarks of aging,' though its importance has been somewhat revised in light of newer epigenetic clocks. Telomerase is the enzyme that lengthens telomeres (it's active in stem cells and cancer cells, but not most somatic cells).
Letter V
VO2 max
The maximum rate of oxygen consumption during intense exercise, measured in milliliters of oxygen per kilogram of body weight per minute (ml/kg/min). VO2 max is the single strongest predictor of all-cause mortality in healthy adults — a higher VO2 max means a lower risk of dying. Formal testing requires a treadmill or bike test with a mask; wearables like Apple Watch and Garmin estimate VO2 max from submaximal exercise data. Zone 2 cardio is the primary training method for improving VO2 max.
Letter W
Wearable
A category of consumer electronic device worn on the body that tracks physiological signals — heart rate, sleep, activity, HRV, SpO2, skin temperature, etc. Categories include smartwatches (Apple Watch, Fitbit, Withings), rings (Oura), straps (Whoop), and patches (CGMs). See our Best Longevity Wearables guide.
Letter Z
Zone 2 cardio
A specific training intensity zone — typically defined as 60-70% of maximum heart rate, or the highest intensity at which you can still hold a conversation. Zone 2 training builds mitochondrial density and efficiency, improves fat oxidation, and raises VO2 max over time. Most longevity clinicians recommend 3-4 hours per week of Zone 2 cardio as a foundation of any cardiorespiratory fitness program. See our Exercise for Longevity guide.