1. The process of doing or performing something: the act of thinking.
2. Something done or performed; a deed: a charitable act.
3. A product, such as a statute, decree, or enactment, resulting from a decision by a legislative or judicial body: an act of Congress.
4. A formal written record of proceedings or transactions.
5. One of the major divisions of a play or opera.
6. 1. A performance or entertainment usually forming part of a longer presentation: a juggling act; a magic act.
   2. The actor or actors presenting such a performance: joined the act in Phoenix.
7. A manifestation of intentional or unintentional insincerity; a pose: put on an act.

1. To play the part of; assume the dramatic role of: She plans to act Lady Macbeth in summer stock.
2. To perform (a role) on the stage: act the part of the villain.
3. 1. To behave like or pose as; impersonate: Don't act the fool.
   2. To behave in a manner suitable for: Act your age.

1. To behave or comport oneself: She acts like a born leader.
2. To perform in a dramatic role or roles.
3. To be suitable for theatrical performance: This scene acts well.
4. To behave affectedly or unnaturally; pretend.
5. To appear or seem to be: The dog acted ferocious.
6. To carry out an action: We acted immediately. The governor has not yet acted on the bill.
7. To operate or function in a specific way: His mind acts quickly.
8. To serve or function as a substitute for another: A coin can act as a screwdriver.
9. To produce an effect: waited five minutes for the anesthetic to act.
10. 1. To perform in or as if in a play; represent dramatically: act out a story.
    2. To realize in action: wanted to act out his theory.
11. To express (unconscious impulses, for example) in an overt manner without conscious understanding or regard for social appropriateness.
12. To misbehave.
13. To malfunction.
14. Informal To become active or troublesome after a period of quiescence: My left knee acts up in damp weather. Her arthritis is acting up again.

1. 1. To perform in or as if in a play; represent dramatically: act out a story.
   2. To realize in action: wanted to act out his theory.
2. To express (unconscious impulses, for example) in an overt manner without conscious understanding or regard for social appropriateness.

1. To misbehave.
2. To malfunction.
3. Informal To become active or troublesome after a period of quiescence: My left knee acts up in damp weather. Her arthritis is acting up again.

Usage Note: The words act and action both mean "a deed" and "the process of doing." However, other senses of act, such as "a decision made by a legislative body" and of action, such as "habitual or vigorous activity" show that act tends to refer to a deed while action tends to refer to the process of doing. Thus, people engage in sex acts but not sex actions. By the same token, a person may want a piece of the action, but not a piece of the act. The demands of meaning or idiom will often require one word or the other. But in some cases either can be used: my act (or action) was premature.