A system of units, now obsolete, in which the fundamental units of length, mass, and time were the centimetre, gram, and second. In order to include and completely define electric and magnetic quantities a fourth fundamental quantity is required. As a result two mutually exclusive systems of CGS units arose: the CGS electromagnetic units (emu) and the CGS electrostatic units (esu).
In the emu system the fourth quantity was the permeability of free space, μ0, which was chosen to be unity. In the esu system the fourth quantity was the permittivity of free space, ε0, also chosen to be unity. The choice of the four fundamental quantities completely defined each system but since, by Maxwell’s equations, it can be shown that
where c is the speed of light, the systems were mutually exclusive. All mechanical quantities however, such as force and work, had the same units in either system. To differentiate between the two systems the prefix ab- was used to denote units in the emu system and the prefix stat- for units in the esu system. The ratio of ab- unit to equivalent stat- unit for the primary CGS units, such as current, was equal to c or 1/c, where c was measured in cm/s. The ratio for secondary units was some power of c.
CGS units were superseded by the MKS system of units, from which the system of SI units was developed.