Converse

1. (Logic) A proposition which arises from interchanging the terms of another, as by putting the predicate for the subject, and the subject for the predicate; as, no virtue is vice, no vice is virtue.
2. (Math.) A proposition in which, after a conclusion from something supposed has been drawn, the order is inverted, making the conclusion the supposition or premises, what was first supposed becoming now the conclusion or inference. Thus, if two sides of a sides of a triangle are equal, the angles opposite the sides are equal; and the converse is true, i.e., if these angles are equal, the two sides are equal.

It should not (as is often done) be confounded with the contrary or opposite of a proposition, which is formed by introducing the negative not or no.