'Progressive Web App (PWA)' is a type of application software delivered through the web, built using common web technologies including HTML, CSS, and JavaScript. It is intended to work on any platform that uses a standards-compliant browser, including both desktop and mobile devices.

Introduced by Google in 2015, PWAs aim to bridge the gap between web applications and native applications. They offer an app-like experience on web pages or websites, and are reliable, fast, and engaging. PWAs can be accessed through a browser but also offer features traditionally available only to native apps, like offline functionality, push notifications, and device hardware access.

Key features of PWAs include the ability to work offline, near-instant loading (thanks to service workers), and the capacity to receive push notifications. They can be added to a user's home screen with an icon, making them readily accessible. PWAs ensure an immersive full-screen experience and can even re-engage users with web push notifications.

The development of PWAs not only improves the user experience but also eases the developer’s workload by using a single code base for all platforms. They are seen as a viable alternative to more costly native app development, providing better performance than traditional web apps.

In conclusion, Progressive Web Apps represent a significant evolution in the web application paradigm. They combine the best of web and mobile apps, offering users a high-quality, unified experience across devices and platforms.