ox·y·mo·ron 
(ŏk′sē-môrŏn′)
(ŏk′sē-môrŏn′)n. pl. ox·y·mo·rons or ox·y·mo·ra (-môrə)
A rhetorical figure in which incongruous or contradictory terms are combined, as in a deafening silence and a mournful optimist.
[Greek (attested only in Latin sources) oxumōron, an expression that is witty because paradoxical, from neuter of *oxumōros, pointedly foolish : Greek oxus, sharp, keen; see OXYGEN + mōros, dull, foolish.]
ox′y·mo·ronic (-mə-rŏnĭk) adj.
ox′y·mo·roni·cal·ly adv.