Gehenna is the Greek γέεννα geenna transliteration of the Hebrew גיינום / גיהנם ge-enom / ge-hinnom, meaning “Valley of [the sons of] Hinnom”, south of Jerusalem, where some of the ancient Israelites “passed children through the fire”, sacrificing them to the Canaanite god Molech [1]. The place is called “Tophet” in Isaiah 30:33. In later years, Gehenna continued to be an unclean place used for burning trash from the city of Jerusalem, notorious for the fact that it continuously smoldered, with smoke and heat, and garbage and decayed animal remains covered with maggots and worms, and occasional outbursts of flame. Jesus used Gehenna as an illustration of hell.[2]
"You serpents, you brood of vipers, how will you escape the judgment of Gehenna? Matthew 23:33.
"And if your eye causes you to sin, pluck it out; it is better for you to enter the kingdom of God with one eye than with two eyes to be thrown into Gehenna, where their worm does not die, and the fire is not quenched. Mark 9:47-48.
The "judgment of Gehenna" is a metaphor for the "judgment of burning", an allusion to the everlasting condemnation to eternal torment in the lake of fire seen in Revelation 14:9-11; 20:10, 14-15.