Ἀβαδδών (Heb. אֲבַדּוֹן, destruction; LXX, ἀπώλεια, only in Wisdom Lit., of the place of the ruined dead: Job.26:6, 28:22, 31:12, Psa.88:12, Pro.15:11*), indecl.;
in NT, Abaddon, the angel of the Abyss: Rev.9:11.†
(From Abbott-Smith. LSJ has no entry)