The final act of the story. It’s important to note that the third act tension is either more focused than or different than the second act tension.
In Star Wars, the second act is about getting the Death Star plans to the Rebels. The third act is about assaulting the Death Star and destroying it. In Fight Club, the second act tension is about building Fight Club and whether if it will be successful. The Third Act turns this objective on its head: It’s about the Protagonist trying to stop the now-destructive and anarchic Fight Club from blowing up the banks.