The device used for converting AC (Alternating Current) to DC (Direct Current) is called a "rectifier." A rectifier is an electrical device that allows current to flow in only one direction, effectively converting AC power from a source like the electrical grid or an alternator into DC power, which is commonly used in electronic devices, batteries, and many other applications. There are different types of rectifiers, including diode rectifiers, bridge rectifiers, and more sophisticated ones like thyristor-based rectifiers and solid-state rectifiers, each serving specific purposes based on the application's requirements.