1. not true to duty or obligation or promises

- an unfaithful lover

Similar word(s): inconstant, apostate, perfidious, punic, treacherous, untrue, untrustworthy, untrusty

2. having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend

- her husband was unfaithful

Similar word(s): adulterous, cheating

3. having the character of, or characteristic of, a traitor

Similar word(s): disloyal, faithless, traitorous, treasonable, treasonous

4. not trustworthy

- an unfaithful reproduction

Similar word(s): inaccurate