Static files as in static file handling in a web server no CGI, modules, server side code required.

Served as "flat files" - filesystem, object store, what have you. No server logic generating content, just passing around of strings and binary data. Files are the representation are the source of truth. Counter to a web app, where the content response is ephemeral and the "source of truth" is scattered across a writeable DB and recombinated (potentially) on every request.

Interesting question though, I (a web dev) just take the term for granted.

It's been always the same. Backend, server logic, database... dynamic content; on one hand. JavaScript runs on the browser for almost anything, sometimes for dynamic content. But it's not tied. You could have an in-browser button with a counter for the numbers of times it was pressed (that's actually an entry-level programmer exercise) and that's a static site. If you saved the counter value to the server (e.g. database) then it's not static anymore.

No, static sites are usually more js heavy because they offset all the processing to the client.

This is one reason I avoid static sites, and just heavily cache the server.