I wouldn't say the term "CSS algorithm" has widespread usage yet, but I think Lara Schenck might be onto something. She defines it as:
a well-defined declaration or set of declarations that produces a specific styling output
So a CSS algorithm isn't really a component where there is some parent element and whatever it needs inside, but a CSS algorithm could involve components. A CSS algorithm isn't just some tricky key/value pair or calculated output — but it could certainly involve those things.
The way I understand it is that they are little mini systems. In a recent post, she describes a situation involving essentially two fixed header bars and needing to deal with them in different situations. In this example, the page can be in different states (e.g. a logged-in state has a
position: fixed; bar), and that affects not only the header but the content area as well. Dealing with all that together is a CSS algorithm. It's probably the way we all work in CSS already, but now have a term to describe it. This particular example involves some CSS custom properties, a state-based class, two selectors, and a media query. Classic front-end developer stuff.