“Acts” was used as a familiar term from Diablo days. In terms of GD it’s a way to talk about content as it was released, though the chunks are somewhat defining.
Act 1 - kill the Warden
Act 2 - kill Crowley
Act 3 - kill the Aetherial (the Amalgamation?)
Act 4 - kill the Chthonian boss Karroz in Darkvale
Act 5 - kill Log
The Grim Dawn wiki, https://grimdawn.gamepedia.com/Grim_Dawn_Wiki, doesn’t end Act 3 with the Amalgamation, but I do. So “acts” are open to interpretation. There is no real formal demarcation of an act like in Diablo or PoE.
There are two more “acts” coming in the expansion.