n.
Function: noun
Date: circa 1681
1 : something of a contrary nature or quality : OPPOSITE , REVERSE
2 : a proposition or theorem formed by contradicting both the subject and predicate or both the hypothesis and conclusion of a given proposition or theorem <the inverse of “ if A then B ” is “ if not-A then not-B ” > ― compare CONTRAPOSITIVE
3 a : INVERSE FUNCTION also : an operation (as subtraction) that undoes the effect of another operation b : a set element that is related to another element in such a way that the result of applying a given binary operation to them is an identity element of the set