The Oxford Thesaurus An A-Z Dictionary of Synonyms - infidelityn.

1) unfaithfulness, faithlessness, treachery, traitorousness, disloyalty, perfidy, falseness, apostasy, heresy: The infidelity of his supporters brought about his downfall. 2) adultery, cuckoldry; affair, liaison, amour, Colloq cheating: Infidelity is a common cause of marriage break-up. She forgave him his many infidelities.