postulate

n. [ L. postulatum a demand, request, prop. p. p. of postulare to demand, prob. a dim. of poscere to demand, prob. for porcscere; akin to G. forschen to search, investigate, Skr. prach to ask, and L. precari to pray: cf. F. postulat. See Pray. ] 1. Something demanded or asserted; especially, a position or supposition assumed without proof, or one which is considered as self-evident; a truth to which assent may be demanded or challenged, without argument or evidence. [ 1913 Webster ]

2. (Geom.) The enunciation of a self-evident problem, in distinction from an axiom, which is the enunciation of a self-evident theorem. [ 1913 Webster ]

The distinction between a postulate and an axiom lies in this, -- that the latter is admitted to be self-evident, while the former may be agreed upon between two reasoners, and admitted by both, but not as proposition which it would be impossible to deny. Eng. Cyc. [ 1913 Webster ]