Act

One of the meanings of "act" usually refers to a sub-part (division or unit) of a theatrical performance, like a play. Relative to Warcraft, the RTS games (starting with Warcraft II: Tides of Darkness) were usually played in series of acts, as if the game were presented like 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. 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.

The Acts
No further official comments on this concept have been made, however the scope of the acts are likely the following:
 * Act One was everything in Northrend up until the Battle of Angrathar the Wrathgate, and its accompanying cinematic.
 * Act Two was everything after that, including the Battle for the Undercity, the Ulduar trailer which was a continuation of the Wrathgate plot, and ending with the battle against Yogg-Saron. Each one of the three visions in Yogg-Saron's mind touched upon a key plotline of the expansion.
 * Act Three was everything after that, including the Argent Tournament, and ending with the battle with the Lich King in Icecrown Citadel.