Unfortunately, the term has been co-opted to mean "website I can install/pin as an app". Again, Google is probably to blame for this, but AFAICT it was never meant to be the meaning of the term. What it does do is create misunderstandings, like a sibling thread claiming that (desktop) Firefox doesn't support PWAs because you can't install anything.

[1] https://developer.mozilla.org/en-US/docs/Web/Progressive_web...

And "progressive enhancement" just means "being written in such a way that you'll work with a core set of APIs — all the way down to 'no javascript support' — but will take advantage of further APIs if they're available." Where the app forms a sort of Russian nesting doll of functionality, where it "launches" with just the innermost layer (e.g. just server-rendered HTML), and then in turn each new layer is enabled if possible (i.e. if Javascript is enabled, the browser will load some JS from a <script> tag that will then take over the DOM and turns the page into a SPA; etc.)

It's just a further refinement of the older concept of "graceful degradation", where the gracefully-degraded experience is delivered at the start, and then bootstrapped out into the non-degraded version.

Under this definition, the "further layers of functionality" provided to "installed" webapps in some browsers are just a specific kind of progressive enhancement. The app detects that it's being run "installed", and takes advantage of that. (And before that, the browser detects that the app has such a layer, and therefore treats the webapp as "installable.")

Thus, all "installable" apps are PWAs. (But not all PWAs are "installable"!)

I would note that webapps like Slack — i.e. webapps that have both web and "native" versions, where the "native" version is just the same remote webapp from the web version, loaded in a special browser that exposes extra capabilities that the webapp then takes advantage of — are also PWAs by this definition.