having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend(adj.all)
Example: Her husband was unfaithful.
source: wordnet30
Not faithful; not observant of promises, vows, allegiance, or duty; violating trust or confidence; treacherous; perfidious; as, an unfaithful subject; an unfaithful agent or servant.(adjective)
source: webster1913
adjective satellite (s)
having the character of, or characteristic of, a traitor(adj.all)