Progressive Web Apps - basically HTML5 web sites that work well on a phone, and take advantage of some newer browser features to make a web page more "app-like." Not sure if there's a distinction to be made between PWA and Single Page Applications (SPA), but in either case, you can use Service Workers, local storage, offline mode etc as well as "app" features like notifications and a homepage shortcut. The experience ends up being very much like that of an app, without a huge install.
So sounds like the promise of web-based "apps" from pre-app store iOS to WebOS to Firefox OS... finally about to be realized thanks to official Google backing. RIP to all the minor players who came before and failed.
