A boss is a computer-controlled opponent which is found in video games. Their purpose is to test the skills that the player has accumulated over the course of a game. Boss battles are generally seen at the climax of a particular section of the game, usually at the end of a stage or level, or guarding a specific objective, and the boss enemy is generally far stronger than the minions the player has faced up to that point. A fight with a boss character is commonly referred to as a boss battle.
The first interactive game to feature a boss was dnd, a 1975 computer role-playing game for the PLATO system.  One of the earliest dungeon crawls, dnd implemented many of the core concepts behind Dungeons & Dragons. The objective of the game is to retrieve an "Orb" from the bottommost dungeon. The orb is kept in a treasure room guarded by a high-level enemy named the Gold Dragon. Only by defeating the Dragon can the player claim the orb, complete the game, and be eligible to appear on the high score list. 
The first arcade game to feature a boss was Phoenix, a fixed shooter developed by Taito in 1980. Phoenix includes five levels ("Rounds") which pit the player against swarms of alien birds. During the first two Rounds, the player is assaulted by the pigeon-like "Scouts", whereas the more formidable "Soldiers" are introduced in Rounds 3 and 4. On disposing these enemies, a giant mothership appears in the fifth and final Round.
Bosses are "super-powered" in comparison with regular enemies, and are usually found at the end of a level or area. Most games also include a "final" boss, which is usually the main antagonist in the story, at the very end of the game. While most games include a mixture of boss opponents and regular opponents, some only have one or the other—for example, Shadow of the Colossus for the PlayStation 2 has no enemies other than bosses.
A "miniboss" or "sub-boss" is an intermediate kind of enemy, typically appearing in the middle of a level or immediately prior to the level's actual boss.
A "boss rush" is a round in which boss characters from earlier in the game are fought in succession, often leading up to a bigger boss.