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Kitto's Popular Cyclopedia of Biblial Literature
The Greek word means literally without bottom, but actually deep, profound. In the New Testament it is used as a noun to describe Hades, or the place of the dead generally (Romans 10:7); but more especially that part of Hades in which the souls of the wicked were supposed to be confined (Luke 8:31; Revelation 9:1-2; Revelation 9:11; Revelation 20:1; Revelation 20:3; comp. 2 Peter 2:4). In the Revelation the Authorized Version invariably renders it 'bottomless pit,' elsewhere 'deep'.
Most of these uses of the word are explained by reference to some of the cosmological notions which the Hebrews entertained in common with other Eastern nations. It was believed that the abyss, or sea of fathomless waters, encompassed the whole earth. The earth floated on the abyss, of which it covered only a small part. According to the same notion, the earth was founded upon the waters, or, at least, had its foundations in the abyss beneath (Psalms 24:2; Psalms 136:6). Under these waters, and at the bottom of the abyss, the wicked were represented as groaning, and undergoing the punishment of their sins. There were confined the Rephaim—those old giants who, while living, caused surrounding nations to tremble (Proverbs 9:18; Proverbs 30:16). In those dark regions the sovereigns of Tyre, Babylon, and Egypt are described by the prophets as undergoing the punishment of their cruelty and pride (Jeremiah 25:14; Ezekiel 28:10, etc.). This was 'the deep' into which the evil spirits in Luke 8:31, besought that they might not be cast, and which was evidently dreaded by them [HADES].
Kitto, John, ed. Entry for 'Abyss'. "Kitto's Popular Cyclopedia of Biblial Literature". https://www.studylight.org/encyclopedias/eng/kbe/a/abyss.html.