Dungeon

Dungeon has multiple meanings in World of Warcraft:
 * 1) A general fantasy term for an adventuring area, often confined, that is inhabited with enemies (tombs, caves, castles, ruins, etc.).
 * 2) Any instance, whether designed for a party or a raid.
 * 3) An instance designed to be completed by a 5-man party. The rest of this article will focus on this sense of the term.

Overview
Dungeons are designed for a party of up to five adventurers. A "valid" party consists of one tank, one healer, and three damage dealers (melee or ranged). Players can travel to the dungeon themselves (or be summoned with a Meeting Stone or warlock's Ritual of Summoning), or they can use the Dungeon Finder tool to queue for specific or random dungeons.

Dungeons contain mostly elite creatures, which are more powerful than normal creatures. Dungeons are more challenging and take longer than quests, but are not as difficult or as time-consuming as raids. All dungeons starting with those from The Burning Crusade feature heroic modes, which are more challenging and contain better loot than the normal counterpart.

Normal dungeon bosses typically drop, , and very rarely quality loot. Heroic dungeon bosses typically drop and  loot.

Completing The Burning Crusade heroic dungeons, Wrath of the Lich King normal and heroic dungeons, and Cataclysm normal and heroic dungeons at the appropriate level used to award Justice Points which were redeemable at vendors for gear. However, this currency was removed along with Valor Points at the end of Mists of Pandaria.

In Patch 6.2 of Warlords of Draenor, a new difficulty called "Mythic Dungeons" was introduced. They are designed to be more challenging than Heroic Dungeons and award far superior, rare-quality loot with a chance at epic-quality loot from the end bosses.

Farming dungeons for loot is one of the main ways for players to acquire entry-level gear for raids.

Megadungeon
The term "megadungeon" has been used to describe Operation: Mechagon, Tazavesh, the Veiled Market and (retroactively) the Return to Karazhan, which are massive 5-man dungeons that use a lockout system similar to a raid.

Infinite dungeon
The term "infinite dungeon" is used to describe Torghast, Tower of the Damned.

Trivia

 * Dungeon is also a melee tileset in Warcraft III.