Boss

boss, n., video game-ism 1. in general usage, a person who exercises control or authority; from Dutch baas "master." 2. (gamerese) in video games, the most powerful enemy on a level. 3. in RPGs, the most powerful enemy in a module. Also level boss, boss of the level. [GJ1]
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Addendum: In classic "side-scroller" video games like Super Mario Bros. or Sonic the Hedgehog, the boss of the level was the villain at the end of each level who had to be defeated before the player could continue on to the next level. More sophisticated video games, like Metal Gear Solid, have expanded on this concept. [GJ1]