Quote Originally Posted by Jaxzan Proditor View Post
Yeah, in this case the term is being used in nuclear physics, so one area where it might pop up is throwing a bunch of alpha particles at another nucleus to see how big it is, as Grey Wolf says. That’s why it’s called cross section, because in that case it’s fairly analogous to the geometric concept. Another might be to see how much of a radioactive mass decays away into another element, which is a little further afield, but still a cross section of sorts.
It's also used in a very similar way in chemical physics, to describe chemical reaction mechanisms. If one chemical particle -- an atom or molecule -- approaches another, a reaction may occur, depending on things like approach vector and the vibrational and rotational states of the particles.