Glossary

By Rick Campbell · Updated 12 Sept 2026 · Sourced to primary literature · Not medical advice

Every term this site uses, defined plainly and in one place. Where a definition is contested or a term is used loosely elsewhere (basal versus resting metabolic rate, for instance), the entry says so rather than picking a side quietly.

58 terms in 6 groups. Each links to the calculator or guide where it does real work.

Body composition

Body mass indexalso: BMI: Weight in kilograms divided by height in metres squared. A screening ratio, not a measure of body fat: it counts total mass without asking what that mass is made of. Adopted by the WHO for international classification in the 1990s and still the standard first-pass screen, with the well-documented blind spots that follow from seeing only two numbers. See it in use.
Body fat percentage: The share of total body mass that is fat tissue, including both essential fat and stored fat. Every practical method of estimating it (tape, calipers, impedance, even DEXA) is an estimate with an error band, and the bands are wide enough that the method should always be named alongside the number. See it in use.
Essential fat: The minimum body fat required for normal physiological function: hormone production, organ protection, temperature regulation and nerve insulation. Usually put at about 2–5% of body mass in men and 10–13% in women. It is a floor, not a target, and health problems appear as people approach it.
Lean body massalso: LBM, fat-free mass: Everything in the body that is not fat: skeletal muscle, bone, organs, skin and a great deal of water. Skeletal muscle is typically only about half of it, which is why day-to-day changes in lean mass are mostly water rather than muscle gained or lost. See it in use.
Fat-free mass indexalso: FFMI: Lean mass divided by height in metres squared, the muscularity equivalent of BMI. The normalised version adds 6.1 × (1.8 − height in metres) so that tall and short people can be compared fairly. It inherits the error of whatever body fat estimate produced the lean mass figure. See it in use.
Relative fat massalso: RFM: A 2018 equation estimating body fat percentage from the ratio of height to waist alone: 64 − 20 × (height ÷ waist) for men, 76 − 20 × (height ÷ waist) for women. Validated against DEXA in US national survey data, where it predicted fat percentage better than BMI. See it in use.
Body density: Mass per unit volume of the whole body, in grams per cubic centimetre. Fat is less dense than lean tissue, so measuring density lets you estimate the proportion of each. Skinfold equations predict density first and then convert it to a fat percentage with the Siri or Brozek equation. See it in use.
DEXAalso: DXA, dual-energy X-ray absorptiometry: A low-dose X-ray scan that maps fat, lean tissue and bone mineral regionally. The practical reference standard for body composition, typically within one to two percentage points of multi-compartment research models, which still means it is an estimate, not a direct measurement of fat.
Bioelectrical impedance analysisalso: BIA: Estimating body composition from the resistance the body offers to a small electrical current, on the basis that lean tissue conducts better than fat. Convenient and the least stable of the common methods: hydration, food, recent exercise and even foot temperature move readings by several points within a day.
Sarcopenia: Age-related loss of skeletal muscle mass and strength. It can hide inside a normal BMI, because muscle lost and fat gained in similar amounts leaves total weight unchanged, one of the reasons BMI alone becomes less informative with age.
Body recomposition: Fat mass falling while lean mass holds or rises, which can leave the scale almost unchanged. It is read from two weigh-ins plus a body-fat method used the same way both times, not from a single morning weight. See it in use.

Fat distribution

Visceral fat: Fat stored inside the abdominal cavity, around the organs. It is metabolically active in ways subcutaneous fat is not (feeding free fatty acids to the liver and promoting insulin resistance and inflammation), which is why waist-based measures predict cardiometabolic risk better than total weight. See it in use.
Subcutaneous fat: Fat stored directly under the skin, the kind a caliper pinches. It makes up the majority of body fat and carries considerably less metabolic risk than visceral fat, particularly when it sits on the hips and thighs rather than the abdomen.
Waist-to-height ratioalso: WHtR: Waist circumference divided by height, in the same units. One threshold covers both sexes and most heights: keep your waist under half your height. UK guidance formally adopted it in 2022, and it is one of the confirmatory measures in the 2025 obesity framework. See it in use.
Waist-to-hip ratioalso: WHR: Waist circumference divided by hip circumference. The WHO flags substantially increased risk at 0.90 or above for men and 0.85 or above for women. It describes the pattern of fat storage (the 'apple' and 'pear' shorthand) rather than the amount. See it in use.
Body roundness indexalso: BRI: A 2013 index that models the torso as an ellipse and reports its roundness from waist and height. Values run from about 1 for a very elongated body shape to 16 or more for a very round one, and large cohort analyses find mortality risk highest at both extremes. See it in use.
A Body Shape Indexalso: ABSI: Waist circumference divided by the waist you would expect from a person's height and weight, so it isolates central fat from overall size. It has no universal clinical cut-off; interpretation is by age- and sex-specific z-scores against a reference population. See it in use.
Multi-measure assessment: The post-2025 approach of confirming excess adiposity with BMI plus at least one direct measure of fat distribution, rather than BMI alone. Where the two disagree (raised BMI with a normal waist, or the reverse), the disagreement is treated as information rather than noise. See it in use.
Preclinical obesity: Excess adiposity with organs still functioning normally: elevated future risk warranting monitoring and support, but not itself a disease. Distinguished in the 2025 Lancet Commission framework from clinical obesity, where organ function is already affected. The distinction requires a clinical assessment. See it in use.
Visceral fat proxy: A tape-based screen for central fat risk using waist-to-height ratio, WHO waist lines and shape indices. It is not a measurement of visceral adipose tissue volume, which requires imaging. See it in use.

Energy and intake

Basal metabolic ratealso: BMR: The energy the body uses at complete rest to run organs, brain and temperature regulation, usually 60–70% of total daily expenditure. Strictly it is measured after an overnight fast, lying still, in a thermoneutral room, which is why most published 'BMR' equations are really predicting resting metabolic rate. See it in use.
Resting metabolic ratealso: RMR: Energy expenditure measured at rest but under less strict conditions than basal metabolic rate. It typically reads a few per cent higher than true BMR. The two terms are used interchangeably in casual writing, which introduces a small but real inconsistency between calculators. See it in use.
Total daily energy expenditurealso: TDEE: Everything the body burns in twenty-four hours: basal metabolism, the thermic effect of food, deliberate exercise and all other movement. It is the number calorie targets hang from, and every estimate of it carries roughly ±10–15% uncertainty because activity multipliers are population averages. See it in use.
Non-exercise activity thermogenesisalso: NEAT: Energy spent on everything that is not sleeping, eating or deliberate exercise: walking, posture, fidgeting, stairs. It varies between similar people by hundreds of kilocalories a day and explains much of what gets called a fast or slow metabolism. See it in use.
Thermic effect of foodalso: TEF, diet-induced thermogenesis: The energy cost of digesting, absorbing and storing what you eat, roughly 10% of intake overall, but higher for protein than for carbohydrate or fat. It is one modest reason higher-protein diets are slightly more forgiving at the same calorie intake.
Metabolic equivalentalso: MET: A multiple of resting energy expenditure, defined as 3.5 millilitres of oxygen per kilogram per minute, about one kilocalorie per kilogram per hour. An activity at 8 METs costs eight times resting. Published MET values are population averages and vary with fitness, technique and terrain. See it in use.
Adaptive thermogenesisalso: metabolic adaptation: The fall in energy expenditure during sustained weight loss that is larger than the loss of tissue alone would predict, driven mostly by reduced spontaneous movement and slightly increased efficiency. Measured effects are typically 5–15%: a real headwind, not the ruined metabolism of internet legend.
Energy balance: The relationship between energy taken in and energy expended. A sustained deficit means weight is lost, a sustained surplus means weight is gained, and neither side of the equation is fixed: intake changes expenditure and expenditure changes appetite.
Calorie deficit: Eating below total daily energy expenditure. The useful range for most people is 300–500 kcal a day, and past about 1% of bodyweight lost per week the composition of that loss deteriorates towards muscle and water. See it in use.
Macronutrientsalso: macros: Protein, carbohydrate and fat: the three components of food that supply energy, at roughly 4, 4 and 9 kilocalories per gram respectively. Calories decide whether weight changes; the macronutrient split shapes what that change is made of and how it feels. See it in use.
Recommended Dietary Allowancealso: RDA: The daily intake judged sufficient to meet the needs of nearly all healthy people in a group. It is set to prevent deficiency, not to optimise performance or body composition, which is why the protein RDA of 0.8 grams per kilogram sits well below the intakes studied for training and for preserving lean mass in a deficit. See it in use.
Acceptable Macronutrient Distribution Rangealso: AMDR: The share of total energy from each macronutrient that the Institute of Medicine associates with a reduced risk of chronic disease while still meeting nutrient needs: broadly 45 to 65 per cent from carbohydrate, 20 to 35 per cent from fat and 10 to 35 per cent from protein in adults. See it in use.
Weight-loss pace: The weekly rate of planned loss, capped on this site at 1% of current bodyweight and further limited by calorie floors. A pace table shows weeks at each allowed speed; it is not a calendar date you will arrive. See it in use.
Reverse diet: A planned raise from a cutting intake back toward estimated maintenance, usually 50 to 150 kilocalories a week, so you learn the new maintenance instead of swinging from a deficit to an untracked surplus overnight. See it in use.
Metabolic age: A marketing label: the age at which a thinner reference person of the same height and sex has a predicted basal metabolic rate closest to yours. It is not a biological age and it is not measured in tissue. See it in use.

Fitness and training

Maximal oxygen uptakealso: VO2 max: The greatest rate at which the body can take in, transport and use oxygen during exercise, expressed in millilitres of oxygen per kilogram of body weight per minute. It is the single best laboratory measure of aerobic capacity, and it predicts all-cause mortality more strongly than most measurements taken in a gym. See it in use.
Maximum heart ratealso: HRmax: The highest heart rate a person can reach during all-out effort. It falls with age and varies widely between people of the same age, which is why every age-based prediction of it carries a standard deviation of roughly ten beats per minute and should be treated as a starting point rather than a personal number. See it in use.
Heart rate reservealso: HRR, the Karvonen method: The difference between maximum heart rate and resting heart rate, used to set training zones as a percentage of that span added back to the resting rate. It tracks percentage of oxygen uptake reserve more closely than a plain percentage of maximum heart rate does, and it adjusts automatically as resting rate falls with training. See it in use.
Lactate threshold: The intensity above which blood lactate begins to accumulate faster than it can be cleared, marking the real physiological boundary between sustainable and unsustainable effort. Heart rate zones are a convenient proxy for it, but the threshold sits at a different percentage of maximum heart rate in every athlete and moves with training. See it in use.
One-repetition maximumalso: 1RM: The heaviest load a person can lift once through a full range of motion with acceptable technique. Training loads are usually prescribed as a percentage of it, and it can be estimated from a lighter set taken to near failure, though every published estimating equation loses accuracy quickly beyond about five repetitions. See it in use.
Physical activity levelalso: PAL: Total daily energy expenditure divided by basal metabolic rate, giving a unitless multiplier for how active a life is. The FAO, WHO and UNU define lifestyle bands in these terms, and the activity factors used by calorie calculators are a simplified, rounded version of the same idea. See it in use.

Growth and pregnancy

Growth reference: A set of curves describing how a particular population of children actually grew, used to place a child against their peers. The CDC charts are a reference in this sense: they describe American children measured in national surveys, which is a different claim from saying that is how children ought to grow. See it in use.
Growth standard: A set of curves describing how children raised under recommended conditions do grow, and therefore how growth should look. The WHO standard for children under five was built this way, from healthy, breastfed children across six countries, which is why it is preferred for infants rather than a national reference.
Extended BMI percentiles: Curves published by the CDC in 2022 that extend the BMI-for-age chart above the 97th percentile, where the original curves were compressed and unstable. They let severe obesity in children and teenagers be tracked as a continuous quantity rather than everyone above the top curve appearing to sit in the same place. See it in use.
Mid-parental heightalso: Target height: An estimate of a child's adult height from the heights of both parents, adjusting by about 13 centimetres for the sex of the child. It explains most of why healthy children are short or tall, but the prediction interval around it is wide enough that it should never be read as a forecast of a specific final height. See it in use.
Gestational weight gain: The weight gained across a pregnancy, for which the Institute of Medicine published ranges in 2009 that depend on pre-pregnancy body mass index. The ranges are associated with outcomes at population level; they are a topic for the midwife or obstetrician tracking a pregnancy rather than a target to hit alone. See it in use.

Measurement and statistics

Percentile: The percentage of a reference population that falls below a given value. A child at the 60th percentile for BMI-for-age has a BMI higher than about 60 of every 100 children of the same age and sex in the reference sample. It describes position, not health. See it in use.
z-scorealso: standard deviation score: How many standard deviations a value sits from the mean of a reference distribution. Zero is the median, 1.645 is the 95th percentile. Growth charts use z-scores internally because they behave better than percentiles at the extremes. See it in use.
LMS method: The transform growth charts use to turn a measurement into a z-score, with three published parameters per age and sex: L for skew, M for the median and S for the spread. It handles the fact that BMI distributions have far more room above the median than below it. See it in use.
Standard error of estimatealso: SEE: The typical size of the error when a prediction equation is applied to an individual. When we say the tape method is accurate to about ±3–4 percentage points, this is the quantity being described, and it is why a single decimal place on such a result is false precision.
Error band: The range within which a true value most likely sits, given the method used. Every estimate on this site prints one, because a number without its uncertainty invites decisions the number cannot support. See it in use.
Validation study: Research that tests a prediction equation against a reference method in a sample separate from the one it was fitted on. An equation that has not been externally validated may simply be describing the population it came from.
Circumference methodalso: tape method: Estimating body fat from tape measurements of the neck, waist and (for women) the hips, using equations developed at the Naval Health Research Center in 1984. Accurate to about ±3–4 percentage points, and unusually consistent because a tape does not care about hydration. See it in use.
Adiposity rebound: The point in childhood, usually around age five or six, when BMI stops falling and begins to rise again. Its timing is why children are assessed against age-specific percentile curves rather than a fixed threshold. See it in use.
Body surface areaalso: BSA: The external surface of the body in square metres, estimated from height and weight. A clinical quantity used for chemotherapy dosing, cardiac index and burn assessment, because metabolism tracks surface area better than mass across body sizes. It has no healthy range. See it in use.
Skinfold caliper: A spring-loaded instrument that measures the thickness of a pinched fold of skin and the fat directly beneath it, at standardised sites. The equations that convert those folds to a body fat percentage are site-specific and population-specific, and the largest source of error is the person doing the pinching rather than the instrument. See it in use.
Hydrostatic weighingalso: Underwater weighing, densitometry: Weighing a person submerged in water to derive whole-body density, then converting density to fat percentage with a two-compartment equation. For decades the reference method against which tape and caliper equations were calibrated, which is why their published error bands are errors against it rather than against truth.
Air displacement plethysmographyalso: BodPod: A sealed chamber that measures body volume from the air it displaces, giving density and then fat percentage by the same two-compartment maths as underwater weighing. Faster and more comfortable than submersion, and repeatable to roughly a percentage point when clothing and hair are controlled.

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Last updated 12 September 2026. Written by Rick Campbell; not medically reviewed. See review status.