What's the difference between 'athlete' and 'mathlete'?

A mathlete is a person who competes in mathematics competitions 

at any level or any age. More specifically, a Mathlete is a student who participates in any of the MATHCOUNTS programs, as Mathlete is a registered trademark of the MATHCOUNTS Foundation in the United States.

As nouns the difference between athlete and mathlete is that athlete is athlete while mathlete is (us) a member of the math team of a primary school or secondary school.

One difference is athlete is a word. but athlete isn't. In fact, I never heard of mathlete before reading this question.

Nami said it well.

One participates in sports competitions while the other participates in maths competitions.