Gehenna n. : The valley of Hinnom, near Jerusalem, where some of the Israelites sacrificed their children to Moloch, which, on this account, was afterward regarded as a place of abomination, and made a receptacle for all the refuse of the city, perpetual fires being kept up in order to prevent pestilential effluvia. In the New Testament the name is transferred, by an easy metaphor, to Hell.

Example:
What's the difference between hades, Gehenna and tartarus?

Taxonomy
Gehenna, Valley of Hinnom, Hell, Hinnom, Christian, the dead, Joshua, in Jerusalem, The Book of Jeremiah, free encyclopedia, fire, the New Testament, Hades, valley of the son of Hinnom, the Christian, 2 Chronicles, gehinnom, Jewish.

Defination from the web:
Gehenna, gehinnam, or gehinnom (Hebrew: גהנום, גהנם, Greek γεεννα) are words used in Jewish and Christian writings for the place where evil ... The name given to Hell in Islam, Jahannam, directly correlates with Gehenna as well. ...