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Definition of contrapositive:
Synonyms for contrapositive:
antonym, antipodes, reverse, contrary, opposite, converse, antithesis, counter, contradictory, antipode, support.
Usage examples:

Every positive in thought has a contrapositive, and the positive and contrapositive are of the same kind.
 "Logic, Inductive and Deductive", William Minto. 
Every positive in thought has its contrapositive.
 "Logic, Inductive and Deductive", William Minto. 
The proposition 'No A is B' is not considered to assert directly, but only implicitly, that 'No B is A. ' In the same way the form 'Either A is B or C is D' ought to be interpreted as meaning directly no more than this, 'If A is not B, C is D. ' It asserts indeed by implication also that 'If C is not D, A is B. ' But this is an immediate inference, being, as we shall presently see, the contrapositive of the original.
 "Deductive Logic", St. George Stock.