For full article see Wikipedia: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 and is generally a system devised around the convenience of the number ten. 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. Because the SI is not static, units are created and definitions are modified through international agreement among many nations as the technology of measurement progresses, and as the precision of measurements improves.