An Act Boss is simply the boss monster you must defeat at the end of each Act.

Diablo IIEdit

In Diablo II They appear in the following order:

1. Andariel
2. Duriel
3. Mephisto
4. Diablo
5. Baal

Their deaths grants you access to the following Act. Depending whether you have Diablo II: Lord of Destruction or not, you have to kill Normal Baal or Diablo to have access to Nightmare, and you have to kill Nightmare Baal or Diablo to have access to Hell.

Diablo IIIEdit

In Diablo III They appear in the following order:

1. The Butcher
2. Belial
3. Azmodan
4. Diablo

As in Diablo II, their deaths grants you access to the following Act. You have to kill Normal Diablo to have access to Nightmare, and you have to kill Nightmare Diablo to have access to Hell and lastly Hell Diablo to have access to Inferno.