inverse function

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A function whose relation to a given function is such that their composite is the identity function. It is often found by interchanging dependent and independent variables.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mathematics A function that does exactly the opposite of another; formally the inverse function of a function exists such that: .

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a function obtained by expressing the dependent variable of one function as the independent variable of another; f and g are inverse functions if f(x)=y and g(y)=x