To take action or do something. She decided to act quickly to prevent further damage.
To perform a role in a play, movie, or television show. He was excited to act in his first major film.
To behave in a specified way. He tends to act shy around new people.
To have an effect or influence. The medicine will act on the symptoms within an hour.
To serve or function in the capacity of someone or something; to perform the duties of. During the summer, he will act as the company’s legal counsel.