implicit function

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

• noun A function whose value can only be computed indirectly from one or more of the independent variables. For example, in the equation x² + y² = 1, y is an implicit function of x.

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

• noun mathematics Any function that is not formulated in a way that the value may be directly calculated from the independent variable