The fourth word often translated hell in Scripture is the Greek word tartaros . It is only found once in the New Testament:

For if God did not spare angels when they sinned, but cast them into hell [tartaros] and committed them to pits of darkness, reserved for judgment (2 Pet. 2:4).

Tartaros is normally thought of as a special prison for certain angels who sinned; therefore, it is not Sheol/Hades or Gehenna. Jude also wrote of angels who are being detained:

And angels who did not keep their own domain, but abandoned their proper abode, He has kept in eternal bonds under darkness for the judgment of the great day (Jude 6).