International System of Units

The International System of Units (abbreviated SI from the French Le Système international d'unités) is the modern form of the metric system. It is the world's most widely used system of units, both in everyday commerce and in science.

The older metric system included several groups of units. The SI was developed in 1960 from the old metre-kilogram-second (mks) system, rather than the centimetre-gram-second (cgs) system, which, in turn, had a few variants.

The SI introduced several newly named units. The SI is not static, but is a living set of standards by which units are created and definitions are modified through international agreement among many nations as the technology of measurement progresses.

The system is nearly universally employed, and most countries do not even maintain official definitions of any other units. A notable exception is the United States of America, which still uses many old units in addition to SI. In the United Kingdom, conversion to metric units is government policy, but the transition is not yet complete. Those countries that still recognise non-SI units (e.g. the US and UK) have redefined their traditional non-SI units in terms of SI units.