contraposition

n. [ Pref. contra- + position: cf. f. conterposition. ] 1. A placing over against; opposite position. [ Obs. ] F. Potter. [ 1913 Webster ]

2. (Logic) A so-called immediate inference which consists in denying the original subject of the contradictory predicate; e. g.: Every S is P; therefore, no Not-P is S. [ 1913 Webster ]