A Phasor Measurement Unit (PMU) is a device used in electrical engineering to measure the electrical waves on an electricity grid. It captures data on voltage and current in real-time, providing precise information about the system's performance. This data helps operators monitor the grid's stability and efficiency. PMUs utilize a technology called synchrophasor measurement, which synchronizes data collection across multiple locations using a global positioning system (GPS). This allows for a comprehensive view of the grid's health, enabling better decision-making and faster responses to potential issues, ultimately enhancing the reliability of the power supply.