Radiant, a.
Etymology: [L. radians, -antis, p. pr. of radiare to emit rays or beams, fr. radius ray: cf. F. radiant. See Radius, Ray a divergent line.]
1. Emitting or proceeding as from a center; [U.S.] rays; radiating; radiate.
2. Especially, emitting or darting rays of light or heat; issuing in beams or rays; beaming with brightness; emitting a vivid light or splendor; as, the radiant sun. Mark what radiant state she spreads. Milton.
3. Beaming with vivacity and happiness; as, a radiant face.
4. (Her.)
3 more senses
Giving off rays; -- said of a bearing; as, the sun radiant; a crown radiant.
5. (Bot.)
Having a raylike appearance, as the large marginal flowers of certain umbelliferous plants; -- said also of the cluster which has such marginal flowers. Radiant energy (Physics), energy given out or transmitted by radiation, as in the case of light and radiant heat. -- Radiant heat, proceeding in right lines, or directly from the heated body, after the manner of light, in distinction from heat conducted or carried by intervening media. -- Radiant point. (Astron.) See Radiant, n., 3.
1 other Webster entry under this spelling
Radiant, n.
1. (Opt.)
The luminous point or object from which light emanates; also, a body radiating light brightly.
2. (Geom.)
A straight line proceeding from a given point, or fixed pole, about which it is conceived to revolve.
3. (Astron.)
The point in the heavens at which the apparent paths of shooting stars meet, when traced backward, or whence they appear to radiate.
Shades of meaning
Fernald lists radiant only as an antonym, under dark.
He lists it without comment; the entries above explain its neighbors.