TITLE: Why do we use the word "regime" in math/science/engineering instead of "region"?
QUESTION [9 upvotes]: For example, I would hear people say in the "regime" of x>3? Does it have a different meaning/origin to the "region" of x>3?
To me, "region" sounds like the more proper term.
Edit: I have heard this usage in a few context in physics, electrical engineering, and machine learning. It usually involves the professors pointing at a 2D graph and articulating about the behavior of a system/model/function in these "regimes".

REPLY [3 votes]: The term "regime" is often used in dynamical systems where there are mutiple competing influences (or terms in the equations of motion) that would each individually cause the agent to act in qualitative different ways. A "regime" doesn't usually have a sharp boundary like $x \geq 3$, but instead refers to a vague region in parameter space where one of the terms dominates all of the other ones, so that you can either ignore the other terms in the equations of motion or treat them perturbatively (hence the analogy that the dominant term is "ruling over" the dynamics, I guess).
The different regimes are usually qualitatively different from each other, and it's typically simpler to approximately analyze the behavior deep within each regime, but there are usually fuzzy boundaries with multiple effects of comparable importance, where you're not clearly within one regime or another.