The centimetre-gram-second system (abbreviated CGS or cgs) is a variant of the metric system of physical units based on centimetre as the unit of length, gram as a unit of mass, and second as a unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways of extending the CGS system to cover electromagnetism. The CGS system has been largely supplanted by the MKS system, based on metre, kilogram, and second. Note that the term cgs is ambiguous, since there are several variants with conflicting definitions of electromagnetic quantities and units. The unqualified term is generally associated with the Gaussian system of units, so this more precise URI is preferred.