[pcordes]

Presumably the usual technical meaning: https://en.wikipedia.org/wiki/Critical_section

i.e. the part protected by a lock() / unlock() in multi-threaded code.

[marcosdumay]

That deserves a place on the CS bad names gallery, just right of "strong type system". There isn't anything even critical about it, just exclusive.

The usage on the context of optimization is much more deserved, although I never heard anybody calling it "section" before, just critical code, critical loop, or stuff like that.

[dingo_bat]

But isn't that exactly what OP meant?