Smith's Bible Dictionary

Apollyon or, as it is literally in the margin of the Authorized Version of (Revelation 9:11) "a destroyer," is the rendering of the Hebrew word Abaddon, "the angel of the bottomless pit." From the occurrence of the word in (Psalm 88:11) the rabbins have made Abaddon the nethermost of the two regions into which they divide the lower world; but that in (Revelation 9:11) Abaddon is the angel and not the abyss is perfectly evident in the Greek.