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.
Hastings' Dictionary of the New Testament 
The Nuttall Encyclopedia 
A dark sunless waste in the nether deeps, as far below earth as heaven is above it, into which Zeus hurled the Titans that rebelled against him; the term was subsequently sometimes used to denote the whole nether world and sometimes the place of punishment.