Definitions of unfaithful:

• adjective:   not trustworthy

  Example: "An unfaithful reproduction"

• adjective:   not true to duty or obligation or promises

  Example: "An unfaithful lover"

• adjective:   having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend

  Example: "Her husband was unfaithful"

• adjective:   deliberately and abominably disloyal or likely to betray trust or confidence