the mathematical rule or function which permits computation of one variable directly from another when an equation relating both variables is given (Ex.: y is an implicit function of x in the equation x + y + 2xy + xy = 0)
See implicit function in American Heritage Dictionary 4
noun
A function whose relation to the variable is given by an equation for which the function has not been solved explicitly. For example, in the equation x2 + y2 = 1, y is an implicit function of x.