A mobile app that isn't really an app at all? Progressive Web Apps promise exactly that and seem to be a big trend at the moment. PWAs are built with web technologies and then form a website that has many of the features of native mobile apps. These include access to the camera, motion sensor, push notifications, caching or offline availability. PWAs are thus a symbiosis of a responsive website and an app. Since progressive web apps are created using web technologies (e.g. HTML5, CSS3 and JavaScript) and web frameworks (e.g. React or Angular), they are easy for us to create.
The disadvantage?
iOS resists PWAs in some things, because they allow you to sneak apps past their App Store (and therefore their control). By the way, this is also the second disadvantage: PWAs are neither found in App Store nor Google Play Store. The install happens over the browser (analogue to bookmarking a website).