What is the meaning of "cascading' in CSS?

"Cascading" in this context means that because more than one stylesheet declaration could apply to a particular piece of HTML, there has to be a known way of determining which specific stylesheet rule applies to which piece of HTML.

The rule used is chosen by cascading down from the more general declarations to the specific rule required. The most specific declaration is chosen.

When I teach CSS, I always tell the students that "cascading style sheets" means something like "fighting style sheets".

One rule tells your H3 tag to be red, another rule tells it to be green -- the rules are contradicting each other, who will win!? Stylesheet deathmatch!

OK maybe that's a slight exaggeration, but it's far more amenable to non-code, non-programming people who are just starting out than any notion of a cascade, or inheritance.

I do of course make sure to tell them that it's not a problem for the style sheets to be fighting each other, that's the way the language was designed.

Håkon Wium Lie (CSS co-creator) defines "cascade" in his PHD-thesis on CSS as "The process of combining several style sheets and resolving conflicts between them" https://www.wiumlie.no/2006/phd/