Adulterous describes sex, or a relationship, between someone who is already married and a person who is not their wife or husband. You hear it in an adulterous affair, adulterous behaviour, an adulterous relationship. It is a formal, rather cold word — the sort you find in law, in church talk, or in careful writing, not in a chat between friends. Friends would say someone is cheating, or having an affair. Though actually, people sometimes stretch it and call any secret sex outside a couple adulterous even when nobody is married; the strict meaning needs a marriage.