Suppose we have a function f(x,y) that is defined this way: $$ f(x,y) = ax + by + cxy + \frac{dxy}{1+ex+fy} $$ Then the first two terms would be linear first-order terms, and the third term will be a second-order cross term. This terminology comes from polynomial approximation, if I'm not mistaken.
But what if my multivariable is not from a polynomial, like the fourth term? That is the more general situation for a two-dimensional function. The problem is that I do not have a good word for it. Most connotations for "multivariate" or "multivariable" are connected to functions that are actually linear/polynomial in each variable (e.g. $d=0$ in my example).
For my specific case, I want to emphasize that the term I am describing is complicated because it's not linear and it's not a cross term. It's simply a complicated function involving two variables.
Do you know a word that is commonly used to describe this kind of multivariate term?