Noah Webster's Dictionary (n.) The infernal regions, described in the Iliad as situated as far below Hades as heaven is above the earth, and by later writers as the place of punishment for the spirits of the wicked. By the later poets, also, the name is often used synonymously with Hades, or the Lower World in general.

Multi-Version Concordance Tartarus (1 Occurrence) 2 Peter 2:4 For if God didn't spare angels when they sinned, but cast them down to Tartarus, and committed them to pits of darkness, to be reserved for judgment; (WEB WEY YLT)