Source : Webster's Revised Unabridged Dictionary (1913)

Unconstitutional \Un*con`sti*tu"tion*al\, a.
   Not constitutional; not according to, or consistent with, the
   terms of a constitution of government; contrary to the
   constitution; as, an unconstitutional law, or act of an
   officer. --Burke. -- {Un*con`sti*tu"tion*al"i*ty}, n. --
   {Un*con`sti*tu"tion*al-ly}, adv.

Source : WordNet®

unconstitutional
     adj : not consistent with or according to a constitution; contrary
           to the U.S. Constitution [ant: {constitutional}]