From BiblePortal Wikipedia

Webster's Dictionary [1]

(1): (n.) Acuteness (of the faculties); sharpness 9wit.

(2): (n.) The quality or state of being bright; splendor; luster; brilliancy; clearness.

King James Dictionary [2]

Brightness n. briteness. Splendor luster glitter.

1. Acuteness, applied to the faculties sharpness of wit as the brightness of a man's parts.

International Standard Bible Encyclopedia [3]

brı̄t´nes  : Used by the King James Version in  Hebrews 1:3 for "effulgence of his glory," as in the Revised Version (British and American) and the American Standard Revised Version. The Greek apaúgasma may mean either "reflection" or "radiation." Patristic usage favors the latter; compare The Wisdom of Solomon 7:26; also the Nicene Creed: "Light of Light," i.e. the Son not only manifests the Father, but is of the same substance. "What emanates from light, must have the nature of light" (Delitzsch).