mid-15c., "shining, bright, shooting or emitting diverging rays of light," later also of heat, from Latin radiantem (nominative radians) "beaming, shining," present participle of radiare "to beam, shine" (see radiation). Of beauty, wit, etc., "sparkling, beaming," attested from c. 1500. Related: Radiantly.
in optics, "point or object from which light radiates," 1727; see radiant (adj.). In astronomy, of meteor showers, "the point in the heavens from which the shooting stars seem to proceed," by 1864.