parameter (n.)

1650s in geometry, in reference to conic sections, from Modern Latin parameter (1630s), from Greek para- "beside, subsidiary" (see para- (1)) + metron "measure" (from PIE root *me- (2) "to measure").

A geometry term until late 1920s when it began to be extended to "measurable factor which helps to define a particular system," hence the common meaning (influenced by perimeter) of "boundary, limit, characteristic factor," common from 1950s. Related: Parametric; parametrical.