G541 - apaúgasma
A light flashing forth (from), radiation, gleam. From a compound of apo and augazo; an off-flash, i.e. Effulgence.
HELPS Word-studies
541 apaúgasma (from 575 /apó, “from” which intensifies 826 /augázō, “shine,” derived from augē/“dawn”) – properly, someone who literally “shines,” (is radiant). 541 (apaúgasma) is only used in Heb 1:3 where it refers to Christ’s eternal radiance – supremely reflecting the effulgent glory of the Godhead. His eternal light breaks through all the darkness that keeps someone in spiritual ignorance (bondage), i.e. every resistance exerted by sin.