parametric equations
From Glossary of Meteorology
A set of equations in which the independent variables or coordinates are each expressed in terms of a parameter.
For example, instead of investigating y = f(x), or F(x, y) = 0, it is often advantageous to express both x and y in terms of a parameter u: x = g(u); y = G(u). The parameter may or may not have a useful geometric or physical interpretation.
