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act

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verb American English pronunciation: act /ækt/ 
Word Forms
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present tense
I/you/we/theyact
he/she/itacts
present participleacting
past tenseacted
past participleacted
 
  1. 1
    [intransitive] to do something for a particular reason or in a particular way
    act in good faith (=believe that you are acting honestly):

    We acted in good faith – we did not know the goods were stolen.

    act out of necessity/desperation/love/compassion/curiosity etc.:

    She claims that she acted out of necessity (=because she had to).

    1. a.
      to do something immediately, especially in order to deal with an urgent problem

      Had the paramedics not acted so swiftly, he would have died.

      act to do something:

      We must act now to prevent further damage to our rainforests.

  2. 2
    [intransitive] if you act for someone, you do something as their representative
    act for:

    Lawyers acting for the family will issue a statement this morning.

  3. 3
    [intransitive] to behave in a particular way, especially a way that is unusual or annoying

    Stop acting stupid!

    act like:

    I've acted like a total idiot.

    act suspiciously/strangely:

    A man was seen acting suspiciously twenty minutes before the robbery.

    act as if:

    Just act as if nothing's happened.

    1. a.
      to behave in a particular way by doing something
      act reasonably/fairly/illegally/responsibly/dishonestly etc.:

      Did the network act reasonably in firing the reporter?

      The court must establish whether the accused acted illegally.

  4. 4
    [intransitive/transitive] to perform in plays or movies

    I've always wanted to act.

    act the part/role (of someone/something):

    Burt Lancaster acted the part of Joseph.

  5. 5
    [intransitive] to start to have an effect

    Don't expect the medicine to act immediately.

    act on:

    The experiment should show us how sugars act on the enamel of the teeth.

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