/ Dictionary / Index R radian: Meaning and Definition of Find definitions for: ra•di•an Pronunciation: (r?'d?-un), [key] ? n. Math. the measure of a central angle subtending an arc equal in length to the radius: equal to 57.2958. Abbr.: rad Random House Unabridged Dictionary, Copyright 1997, by Random House, Inc., on Infoplease. See also: radian (Thesaurus) radial triangulation radiance Related Content Daily Word Quiz: abscond Analogy of the Day: Today?s Analogy Spelling Bee: Today?s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language