At the end of a level in a video game you typically fight a much larger enemy with much more health, called a boss. Typically these bosses are defeated by figuring out their pattern of attack.
See Also: mini-boss
"The boss fights in Metroid Prime kick ass!"
The big, mean, cheapass mofo you fight at the end of a video game, usually fighting games. Most infamous of the lot are the SNK bosses of KOF and Fatal Fury etc.