The defining features of a progressive web app (PWA) include responsiveness, connectivity independence, and app-like experiences. PWAs are designed to work seamlessly across devices and screen sizes, providing a native-like experience within web browsers. They can function offline and cache data, ensuring users can access content without a stable internet connection. Additionally, PWAs leverage service workers for background synchronization, push notifications, and fast load times. These features enhance user engagement and retention, making PWAs an appealing choice for developers and businesses aiming to reach a broad audience.