(Hebrew אבדון) destruction.
"Sheol (hell) and Abaddon are never full; so the eyes of man are never satisfied." - Proverbs 27:20
"And they had a king over them, which is the angel of the bottomless pit, whose name in the Hebrew tongue is Abaddon, but in the Greek tongue hath his name Apollyon." - Revelation 9:11
"[from the verbal root 'abad to perish, be cut off] Destruction, abyss; the region of the dead, synonym of She'ol in the Old Testament. Equivalent to the Greek apollyon (destruction, laying waste -- Rev 9:11). Thus Abaddon, Apollyon, Hades, and Orcus all signify the underworld -- the kama-loka or region of disintegrating "shells," human or other." - The Theosophical Glossary