1. Not true to duty, obligation or promises
   "an unfaithful lover"
    
2. Having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend
   "her husband was unfaithful"
    
3. Having the character of, or characteristic of, a traitor
   "the unfaithful Benedict Arnold";
   - faithless, traitorous, treasonable, treasonous
    
4. Not trustworthy
   "an unfaithful reproduction"

See also: adulterous, apostate, cheating, disloyal, faithfulness, fidelity, inaccurate, inconstant, perfidious, punic, treacherous, two-timing, untrue, untrustworthy, untrusty

Antonym: faithful

Encyclopedia: Unfaithful