Comment by derefr

In general, the "Progressive" in "Progressive Web App" just means "progressively-enhanced." In conventional usage around most of the internet, any web app that supports any kind of progressive enhancement, is a PWA.

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.