The extremely nasty bad guy you have to fight at the end of a level in various genre of video games. Put there just to piss you off after having completed a very difficult level I guess. Some games have sub-bosses aswell half way through the levels, and almost all have a big nasty boss at the end of the game (sometimes several). R-Type has some of the coolest bosses in history, and Treasure can always be relied on to make some right nasty bosses (in fact, Alien Soldier is almost all bosses). Bosses are found in shoot 'em ups, platformers, beat 'em ups, and sometimes even puzzle games(!).

Sometimes called guardians.