Glossary of Measurement Units

Fundamentals

SI (Système International d'Unités): The International System of Units, the modern form of the metric system. It defines seven base units — meter, kilogram, second, ampere, kelvin, mole, and candela — from which all other SI units are derived.
Base unit: A unit that is defined by convention rather than derived. The seven SI base units form the foundation of the entire measurement system.
Derived unit: A unit formed by combining base units. For example, the newton (N) is defined as kg·m/s².
Prefix: A word element attached to a unit to indicate a multiple or submultiple. SI prefixes include kilo (10³), mega (10⁶), giga (10⁹), milli (10⁻³), micro (10⁻⁶), and nano (10⁻⁹).

Exact conversion: A conversion whose factor is fixed by definition, not measurement. For example, 1 in = 2.54 cm exactly.
Approximate conversion: A conversion whose factor depends on a measured constant with finite precision.
Significant figures: The digits in a number that carry meaningful information about its precision. Trailing zeros after a decimal point are significant; leading zeros are not.
Precision: How close repeated measurements are to each other. Distinct from accuracy.
Accuracy: How close a measurement is to the true or accepted value. Distinct from precision, which measures repeatability.

Meter (m): The SI base unit of length. Since 1983, defined as the distance light travels in a vacuum in 1/299,792,458 of a second.
International mile (mi): A unit of length equal to exactly 1,609.344 meters. Fixed by the 1959 international yard and pound agreement.
Nautical mile (nmi): A unit of length equal to exactly 1,852 meters. Originally defined as one minute of arc of latitude along a meridian.

Kilogram (kg): The SI base unit of mass. Since 2019, defined by fixing the Planck constant to exactly 6.62607015 × 10⁻³⁴ J·s.
Pound (lb): The international avoirdupois pound is defined as exactly 0.45359237 kilograms.
Stone (st): A British unit of mass equal to exactly 14 pounds (about 6.35 kg).

Liter (L): A non-SI unit of volume accepted for use with SI. Exactly 1 cubic decimeter (0.001 m³).
US liquid gallon (gal): A US customary unit of volume equal to exactly 3.785411784 liters, defined as 231 cubic inches.
Imperial gallon (imp gal): A UK unit of volume equal to exactly 4.54609 liters.

Celsius (°C): A temperature scale where 0 °C is the freezing point of water and 100 °C is the boiling point at 1 atm. The SI accepts Celsius for use with SI; 0 °C = 273.15 K.
Fahrenheit (°F): A temperature scale where 32 °F is the freezing point of water and 212 °F is the boiling point at 1 atm.
Kelvin (K): The SI base unit of thermodynamic temperature. Defined since 2019 by fixing the Boltzmann constant to exactly 1.380649 × 10⁻²³ J/K.

Bit (b): The smallest unit of digital information, representing one binary state (0 or 1). The base unit for digital storage conversions.
Byte (B): A unit of digital information equal to exactly 8 bits.
Megabyte (MB): A decimal SI unit of digital information equal to exactly 1,000,000 bytes (10⁶).
Mebibyte (MiB): A binary IEC unit of digital information equal to exactly 1,048,576 bytes (2²⁰).

Joule (J): The SI derived unit of energy, equal to 1 kg·m²/s².
Calorie (cal): A unit of energy. The thermochemical calorie is exactly 4.184 joules.
Watt (W): The SI derived unit of power, defined as one joule per second (1 W = 1 J/s).
Horsepower (hp): A unit of power. The mechanical horsepower is exactly 550 ft·lb/s ≈ 745.7 W.

Pascal (Pa): The SI derived unit of pressure, equal to 1 N/m².
Bar: A metric unit of pressure equal to exactly 100,000 Pa.
PSI: Pounds per square inch, a customary unit of pressure equal to approximately 6,894.76 Pa.