Quick Definition
ἀπαύγασμα, ατος, τό n.n. a light flashing forth, radiation, gleam
Strong's Definition
from a compound of 575 and 826; an off-flash, i.e. effulgence
KJV Usage: brightness
UBS Dictionary of the Greek New Testament
noun, n. | -τος | αὐγάζω
14.48 · Physical Events and States · Light
radiance, reflection shining, either in the sense of radiance from a source or the reflection of a source of light It is impossible to determine whether ἀπαύγασμα should be interpreted in an active sense and therefore ‘radiance,’ or in a passive sence and therefore as ‘reflection.’ If the meaning is ‘radiance,’ then one may translate this phrase in HEB 1:3 as ‘he shines with God’s glory’ or even ‘God’s glory shines through him.’ If one understands the meaning of ἀπαύγασμα as being ‘reflection,’ then one may often say ‘God’s glory shines back from him.’
UBS Dictionary of the Greek New Testament © United Bible Societies, licensed CC BY-SA 4.0.
Thayer's Greek Lexicon
ἀπ-αύγασμα, -ros, τό, (fr. ἀπαυγάζω to emit brightness, and this fr. αὐγή brightness; cf. drockiacpa, ἀπείκασμα, ἀπεικόνισμα, ἀπήχημα), reflected brightness: Christ is called in Heb. i. 3 ἀπαύγ. τῆς δόξης τοῦ θεοῦ, inasmuch as he perfectly reflects the majesty of God; so that the same thing is declared here of Christ metaphysically, which he says of himself in an ethical sense in Jn. xii. 45 (xiv. 9): 6 θεωρῶν ἐμὲ θεωρεῖ τὸν πέμψαντά pe. (Sap. vii. 26 ; Philo, mund. opif. § 51; plant. Noé § 12; de concup. ὃ 11; and often in eccl. writ.; see more fully in Grimm on Sap.1.c., p. 161 54.) [Some interpreters still adhere to the signif. effulgence or radiance (as distinguished from refulgence or reflection), see Kurtz ad loc. ; Soph. Lex. s. v.; Cremer s. y.]
Mounce Concise Greek Dictionary
ἀπαύγασμα apaugasma 1x
a radiance, Heb. 1:3*
Abbott-Smith Greek Lexicon
**† ἀπ-αύγασμα, -τος, τό
(< αὐγή, brightness, whence ἀπαυγάζω, to radiate or reflect), [in LXX: Wi 7:26*;]
of light beaming from a luminous body, radiance, effulgence: He 1:3.†
Liddell-Scott-Jones — Greek-English Lexicon
ἀπαύγασμα (apaugasma) — radiance. ἀπαύγ-ασμα, ατος, τό, radiance, effulgence, of light beaming from a luminous body, φωτὸς ἀϊδίου[LXX+NT+1st c.AD+]