I'm studing C. Petit's work "Faster algorithms for isogeny problems using torsion point images" ([link](https://eprint.iacr.org/2017/571.pdf)) and he talks about meet-in-the-middle approach/strategy for solve some isogenies problems. 

Well, what does Petit mean with meet-in-the-middle approach/strategy? 

I've read that it's an approach that allows you to reduce the complexity of a problem with the aim of using brute force to solve it, but what does it mean in mathematical language?


Any help will be greatly appreciated.