do something1 [intransitive] to do something for a particular purpose or in order to deal with a situationIt is vital that we act to stop the destruction of the rainforests.The girl's life was saved because the doctors acted so promptly.He claims he acted in self-defense.
behave2 [intransitive] to behave in a particular way + adverbJohn's been acting very strangely lately. act like someone/somethingStop acting like spoiled children! act as if/though…She was acting as if she'd seen a ghost.help In spoken English people often use like instead of as if or as though in this meaning: She was acting like she'd seen a ghost.
pretend3 [intransitive] to pretend by your behavior to be a particular type of person + adjectiveI decided to act dumb.He acts all macho, but he's a real softie underneath.
perform in play/movie4 [intransitive, transitive] to perform a part in a play or movieHave you ever acted?Most of the cast act well. act somethingWho's acting (= taking the part of) Hamlet?The play was well acted.
perform function5 [intransitive] to perform a particular role or function act as somethingCan you act as interpreter?A five-year sentence should act as a deterrent to others. act like somethinghormones in the brain that act like natural painkillers
have effect6 [intransitive] act (on something) to have an effect on somethingAlcohol acts quickly on the brain.It took a few minutes for the drug to act.
act/play the fool
to behave in a stupid way in order to make people laugh, especially in a way that may also annoy themQuit playing the fool and get to work!act the foolplay the fool
be/act your age
to behave in a way that is suitable for someone of your age and not as though you were much youngerIsn't it time you started acting your age?be your ageact your age