a ‧ dul ‧ ter ‧ y /əˈdʌlt ə ri/ BrE AmE noun [uncountable]
[ Date: 1400-1500 ; Language: Old French ; Origin: avoutrie , from Latin adulterium , from adulter 'adulterer' , from adulterare ; ⇨ ↑ adulterate ]
sex between someone who is married and someone who is not their wife or husband:
She had committed adultery on several occasions.
—adulterous adjective