I'm studing Petit's work "Faster algorithms for isogeny problems using torsion point images" 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

I'm studing C. Petit's work "Faster algorithms for isogeny problems using torsion point images" (link) 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.