To act means to do something or take action. When a problem occurs, someone needs to act to solve it. When a fire alarm sounds, students act by calmly walking to the exits. Acting is the opposite of just thinking or talking about something: it's the moment you actually do it.

The word also means to behave in a particular way. When someone acts nervous before a presentation, they might fidget or speak quickly. Your parents might tell you to act your age, meaning to behave maturely. When someone acts like they know everything, they're behaving as if they're an expert even if they aren't.

In theater and film, to act means to perform as a character in a play or movie. Actors memorize lines, express emotions, and pretend to be someone else. Good acting makes audiences forget they're watching a performance. A student might act in the school play, or a professional actor might spend months preparing for a role.

An act can also be a noun meaning a single thing you do, like an act of kindness when you help someone. In a play, an act is a major section of the story, like Act One or Act Two. Laws passed by Congress are called acts, such as the Civil Rights Act of 1964.