We're using "relational operator function", too, and quite a few places had to be fixed to avoid the term "comparison operator function". I'd mildly favor keeping the term "comparison operator function", since we're actually talking about a certain subset of operator functions here.
Adding a definition in [over.binary] is a good idea; maybe we should also define "relational operator function" and "equality operator function" there.

I think the term should be "comparison operator function"; it's a special kind of operator function. "comparison function" sounds a bit like it would include memcmp.