Source : Webster's Revised Unabridged Dictionary (1913)

Adversative \Ad*ver"sa*tive\, a. [L. adversativus, fr.
   adversari.]
   Expressing contrariety, opposition, or antithesis; as, an
   adversative conjunction (but, however, yet, etc. ); an
   adversative force. -- {Ad*ver"sa*tive*ly}, adv.

Adversative \Ad*ver"sa*tive\, n.
   An adversative word. --Harris.

Source : WordNet®

adversative
     adj : expressing antithesis or opposition; "the adversative
           conjunction `but' in `poor but happy'" [syn: {oppositive}]