Gehenna: the place of final, fiery divine judgment on the wicked
The word is not really Greek at all. It is a Greek spelling of an Aramaic name (Gehinnam) for a real place near Jerusalem, the Valley of Hinnom, a deep ravine just south and west of the old city. In the Old Testament that valley was notorious: it was where some of Judah's kings burned their own children in sacrifice, and the prophet Jeremiah declared that God would turn it into a "Valley of Slaughter" piled with corpses. Over the centuries between the Testaments, Jewish writers took this disgraced, fire-and-death-laden valley and made its name a label for the place where God would finally judge the wicked. By the time of Jesus the name no longer pointed to a spot on a map; it pointed to the future reckoning itself.
In the New Testament the word always carries this single meaning. It is the place of final judgment, regularly tied to fire, and the one with authority to send people there is God, never any human court and never the devil. Jesus uses it as the gravest of warnings: better to lose a hand or an eye than for the whole self to be thrown into Gehenna. James uses it once more, picturing the destructive fire of human speech as flaming up from Gehenna, treating the name as the very source of ruinous fire.
What the name leaves open is the nature of that judgment. The New Testament does not spell out whether the punishment is conscious torment without end, the total destruction of the wicked, a purifying correction, or a warning that also pointed to the historical ruin of Jerusalem. Faithful scholars read it in each of these ways, and the word itself does not settle the question. What it does say plainly is that a real, future, and dreadful judgment by God awaits, named after the most accursed place the readers' own history could supply.