1650s in geometry, from Modern Latin parameter (1630s), from Greek para- "beside, subsidiary" (see para-(1)) + metron "measure" (see meter, n.2).A geometry term until 1920s when it yielded sense of "measurable factor which helps to define a particular system" (1927). Common modern meaning (influenced by perimeter) of "boundary, limit, characteristic factor" is from 1950s. Related: Parametric.