To act means to do something, especially with a purpose, like when you make a choice or take action. If someone says "Act quickly!" they mean you should do something fast. You can act in a play, pretending to be someone else, which is what actors do. Plays and performances are often divided into parts called acts, like "Act 2 of the play". An act can also mean a kind thing you do, like an act of kindness, or something bad, like an act of anger. Even magicians or circus performers call their shows "acts", like a magic act. Examples: • You should act now before it's too late. • He dreams of learning how to act in movies. • The play was so good, especially the first act. • The magician’s act amazed everyone at the party. • Don’t act like you didn’t know about the surprise!