What is Act?

Act is a major division of the action of a play. A classical play consists of five acts. The number was cut to four by Ibsen. Most modern plays use three acts and many have only two, or they eliminate act structure entirely and use only scene divisions. Acts are sub divided into scene, each of which comprises a unit of action in which there is no change of place or break in the continuity of time. 

Nowadays the end of a scene is indicated by dropping the curtain. The division into acts was brought into English drama by the Elizabethan writers, who followed their Roman models by writing plays of five acts.