Act
The Warcraft RTS games (starting with Warcraft II: Tides of Darkness) were usually played in series of acts. An Act is a sub-part of a theatrical performance.
In World of Warcraft
Jeffrey Kaplan stated in an IGN interview that World of Warcraft: Wrath of the Lich King would have three acts. The first act culminates at Angrathar the Wrathgate. Acts two and three will be patched in. The point of this is to drive the story arc of the entire expansion.[1] Arthas will be come up against for the first time at the Wrathgate, but the final encounter with him won't be until the final patch (and act) of Wrath of the Lich King, some time after release.[2]
No further official comments on this concept have been made, However, Act One was from the arrival in Northrend to the Battle of Angrathar the Wrathgate, and Act Three ended with the final battle against the Lich King in Icecrown Citadel. Act Two's scope is unknown.