1) reflected brightness
1a) of Christ in that he perfectly reflects the majesty of God
2a) shining forth, of a light coming from a luminous body (Vine)
2b) out-raying (Vincent)
Part of Speech: noun neuter
Citing in TDNT: 1:508, 87
From a compound of G575 and G826; an off flash, that is, effulgence: - brightness.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root ἀπαύγασμα; click on the second column to search for that grammatical form of the root ἀπαύγασμα; click on the third column to search for that word and grammatical form; click on the total to search for the root ἀπαύγασμα.