Measure, exactly.

A unit is a fixed quantity — set by tradition, trade practice or law — that other quantities are measured against. Saying a beam is 4 meters long means it is four times the length agreed on as one meter, so the number only carries meaning once the unit is named.

Measurement grew up locally. Egyptian cubits, Roman paces, Chinese chi, English feet and Spanish varas were all tied to bodies, farm work or royal decrees, which is why a mile in one place differed from a mile a few hundred kilometres away. The metric system, created in revolutionary France in the 1790s, was the first attempt at a coherent decimal alternative built on natural constants rather than custom.

Today the global standard is the International System of Units (SI), the modern form of the metric system. It rests on seven base units — metre, kilogram, second, ampere, kelvin, mole and candela — each now defined through fixed physical constants such as the speed of light, so a measurement can be reproduced in any laboratory without a reference artefact.

SI is intended for worldwide use but adoption is uneven. United States customary units still dominate everyday American life, the UK mixes metric with miles and pints, aviation measures altitude in feet, shipping uses nautical miles and knots, and astronomers work in light years and parsecs. Engineering and trade routinely require moving between these systems.

Unitspeak exists to make that movement quick and dependable: convert between units within and across systems, see the exact factor behind every result, and read how each family of units relates to the SI base it is derived from.