[Proj] Question for the math types

[Gerald I. Evenden]

Several projections are in the form f(phi,lam,t) where t is a third equation 
typically requiring Newton-Raphson for solution.

Question: what is the name of the function of t?

My initial reaction is to call it a parametric but I suspect that is not 
correct.
-- 
The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist