The key feature of a Progressive Web App (PWA) is its ability to provide a native app experience while leveraging web technologies. PWAs are designed to be fast, reliable, and engaging, functioning seamlessly across devices. They utilize service workers to cache resources and ensure offline capabilities, which allows users to access the app even without an internet connection. PWAs also support features such as push notifications and home screen installation, enhancing user engagement and retention. Their responsiveness and low data usage make them an attractive alternative to conventional mobile applications.