I prefer a more direct formulation of what mathematics is, rather than what it is about.

In that case, mathematics is a demonstration of what is apparent, up to but not including what is directly observable.

This separates it from historical record, which concerns itself with what apparently must have been observed. And it from literal record, since an image of a bird is a direct reproduction of its colors and form.

This separates it from art, which (over-generalizing here) demonstrates what is not apparent. Mathematics is direct; art is indirect.

While science is direct, it operates by a different method. In science, one proposes a hypothesis, compares against observation, and only then determines its worth. Mathematics, on the contrary, is self-contained. The demonstration is the entire point.

3 + 3 = 6 is nothing more than a symbolic demonstration of an apparent principle. And so is the fundamental theorem of calculus, when taken in its relevant context.