ad·ver·sa·tive 
(ăd-vûrsə-tĭv)
(ăd-vûrsə-tĭv)adj.
Expressing antithesis or opposition: the adversative conjunction but.
n.
A word that expresses antithesis or opposition.
[Latin adversātīvus, from adversātus, past participle of adversārī, to oppose, from adversus, against; see ADVERSE.]
ad·versa·tive·ly adv.