From BiblePortal Wikipedia

Vine's Expository Dictionary of NT Words [1]

1: Ἀπαύγασμα (Strong'S #541 — Noun Neuter — apaugasma — ap-ow'-gas-mah )

"radiance, effulgence," is used of light shining from a luminous body (apo, "from," and auge, "brightness"). The word is found in  Hebrews 1:3 , where it is used of the Son of God as "being the effulgence of His glory." The word "effulgence" exactly corresponds (in its Latin form) to apaugasma. The "glory" of God expresses all that He is in His nature and His actings and their manifestation. The Son, being one with the Father in Godhood, is in Himself, and ever was, the shining forth of the "glory," manifesting in Himself all that God is and does, all, for instance, that is involved in His being "the very image of His substance," and in His creative acts, His sustaining power, and in His making purification of sins, with all that pertains thereto and issues from it.

Webster's Dictionary [2]

(n.) The state of being effulgent; extreme brilliancy; a flood of light; great luster or brightness; splendor.