Gehenna is the Greek form of the Hebrew Ge Hinnom, or “Valley of Hinnom.”
This was a valley west and south of Jerusalem where children were sacrificed in flames to the Ammonite god Moloch from the tenth through seventh centuries B.C.
In Jewish and Christian thought, it became another name for the place where the wicked burn after death.
It is mentioned in both the New Testament and the Talmud.