Boss
Also said as Boss fight, End-of-level boss
A big set-piece enemy that ends a stretch of the game and tests what the stretch taught you. Usually it has phases: as its health falls, it changes how it fights.
It gives a section of the game a shape and a finish, and asks you to prove what you have learned. A boss that is only a tall health bar teaches nothing and just takes a long time.
Say it
“Three phases: at 66% health, it adds the high swing the player learned in the last room. At 33% the tells get a third shorter. Below that, no new moves.”
Where you’ve seen it
Super Mario Bros. ends every castle with Bowser, but the first seven are impostors in disguise. Only World 8-4 has the real one. Shadow of the Colossus is nothing but boss fights: each colossus is a climbing puzzle you solve on its body.
Not to be confused with tough enemy
A tough enemy is a harder version of what you have been fighting. A boss has its own rules, its own arena, and usually asks a new question.