/ Dictionary / Index R radian: Meaning and Definition of Find definitions for: ra•di•an Pronunciation: (rā'dē-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: mamba Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language