unfaithful - definition and synonyms

adjective 

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/ʌnˈfeɪθf(ə)l/
  1. 1
    having a sexual relationship with someone who is not your husband, wife, or partner
    unfaithful to:

    He had never been unfaithful to her before.

  2. 2
    literary not loyal

derived word

unfaithfulness

noun [uncountable]