Glossary
Definitions, formulas, and examples for common measurement units.
Parsec
A unit of distance equal to about 3.26 light-years.
AstronomyLight-year
The distance light travels in one Julian year — about 9.461 × 10¹⁵ metres.
AstronomyAstronomical Unit
The mean distance from Earth to the Sun — exactly 149,597,870,700 metres.
Energy & NutritionCalorie
A unit of heat energy. In nutrition, 1 Calorie (kcal) = 4,184 joules.
TemperatureKelvin
The SI base unit of thermodynamic temperature. 0 K is absolute zero.
EnergyBTU (British Thermal Unit)
The energy needed to raise 1 pound of water by 1 °F — approximately 1,055 joules.
SoundDecibel
A logarithmic unit for ratios of power or amplitude. 0 dB is the threshold of human hearing.
PressurePascal
The SI unit of pressure — 1 newton per square metre.
SpeedKnot
A unit of speed equal to one nautical mile per hour — about 1.852 km/h.
AreaHectare
A metric unit of area equal to 10,000 m² or about 2.471 acres.
DistanceNautical Mile
A unit of distance equal to exactly 1,852 metres, based on one arcminute of latitude.
ForceNewton
The SI unit of force — the force needed to accelerate 1 kg at 1 m/s².
PowerWatt
The SI unit of power — 1 joule per second.
PowerHorsepower
A traditional unit of power. 1 metric hp = 735.5 W; 1 US hp = 745.7 W.
SpeedMach Number
The ratio of an object's speed to the local speed of sound (~343 m/s in dry air at 20 °C).