In its main modern use, unfaithful describes someone who has sex or a romantic relationship with a person who is not their husband, wife, or partner. The usual pattern is 'be unfaithful to someone': 'he was unfaithful to his wife'. It is the careful, slightly formal way to say it. Everyday English prefers 'cheat on' — 'he cheated on his wife' — which is what friends actually say to each other. 'Unfaithful' is what a newspaper, a lawyer, a novel, or a person choosing their words gently would use. It sounds sadder and less angry than 'cheat', and it puts the focus on a promise that was broken rather than on the act. There is an older, wider meaning still alive in books and speeches: not loyal, not keeping faith with a person, a country, a cause, or a religion — 'unfaithful to his principles', 'an unfaithful servant'. And a third, quite different one for translations and copies: a version that does not follow the original closely is 'unfaithful to the text'. Same idea underneath — something that should have stayed true to something else, and did not. Two useful relatives: the noun is 'unfaithfulness', though 'infidelity' is far more common in serious writing, and the opposite 'faithful' is very much alive in wedding vows ('to be faithful to you').