Personally I’d call it a bit of both. What a lot of people don’t seem to understand about mathematics is how often it changes. It’s been “under development” for the last several thousand years, and it’s not finished yet.

To be honest I think rather than an “invention” or “description” I’d actually call it a language that we use to discuss objective truths about the universe we live in. Like all languages it has its own syntax and grammar, but also like all languages it can change as the context in which it exists changes.