Unfaithful is an adjective that describes someone who is disloyal, unreliable or deceitful. Synonyms for the word unfaithful include a range of terms to describe someone who is not faithful or loyal, such as faithless, disloyal, untrustworthy, treacherous, unreliable, false, undependable, deceitful, perfidious, insincere or deceptive. The word unfaithful is often used in reference to infidelity in relationships, but can also be applied to other areas where someone has broken a promise or trust. No matter the context, unfaithful is a negative descriptor that suggests a lack of integrity and dependability, qualities that are highly valued in personal and professional relationships.