In the world of video-games, a huge area, usually indoors, which houses multiple rooms, puzzles and enemies. The player must navigate his way through the dangerous place and face the dungeon boss (much harder than the regular dungeon enemies), to receive a reward and progress further into the story.
The most famous dungeons are the Legend of Zelda ones, where you can earn a new weapon/item/etc. at some point in the dungeon and you usually face a mini-boss (with difficulty between one of a regular enemy's and the boss's)
Gamer: Finally, I completed that dungeon!