A broadcast receiver is a component in the Android operating system that listens for and responds to broadcast messages from other applications or the system itself. These broadcasts can signal various events, such as changes in network connectivity, battery status, or incoming SMS messages. When a broadcast receiver detects a relevant message, it can trigger specific actions, like starting a service or displaying a notification. Broadcast receivers are defined in the app's manifest file, where developers specify the types of broadcasts they want to listen for. They can operate in the background, allowing apps to respond to events even when they are not actively running. This makes them essential for creating responsive and efficient applications in the Android ecosystem.