Source : Webster's Revised Unabridged Dictionary (1913)

Act \Act\, v. i.
   1. To exert power; to produce an effect; as, the stomach acts
      upon food.

   2. To perform actions; to fulfill functions; to put forth
      energy; to move, as opposed to remaining at rest; to carry
      into effect a determination of the will.

            He hangs between, in doubt to act or rest. --Pope.

   3. To behave or conduct, as in morals, private duties, or
      public offices; to bear or deport one's self; as, we know
      not why he has acted so.

   4. To perform on the stage; to represent a character.

            To show the world how Garrick did not act. --Cowper.

   {To act as} or {for}, to do the work of; to serve as.

   {To act on}, to regulate one's conduct according to.

   {To act up to}, to equal in action; to fulfill in practice;
      as, he has acted up to his engagement or his advantages.