It's not a precise definition (and I'm not sure how actionable it is) but it is a term of art. https://en.wikipedia.org/wiki/Mathematical_maturity

Some people are better with math than others. Undergrad math majors thought the major was pretty easy. Many of us in engineering majors--who weren't that bad at math (it was a good school)--could no more have graduated with a degree in math (or physics) than have flapped our arms and flown.