Source : Webster's Revised Unabridged Dictionary (1913)

   Note: In some of the States, and for some purposes, a woman
         attains lawful age at eighteen. --Abbott.

   Syn: Legal; constitutional; allowable; regular; rightful.

   Usage: {Lawful}, {Legal}. Lawful means conformable to the
          principle, spirit, or essence of the law, and is
          applicable to moral as well as juridical law. Legal
          means conformable to the letter or rules of the law as
          it is administered in the courts; conformable to
          juridical law. Legal is often used as antithetical to
          equitable, but lawful is seldom used in that sense. --
          {Law"ful*ly}, adv. -- {Law"ful*ness}, n.

Source : WordNet®

lawfulness
     n : the quality of conforming to law [ant: {unlawfulness}]