In general, a controller is a device, program, or system that manages or directs the operation of another device, program, or system.
In the context of electronics and engineering, a controller is a device that regulates or controls the behavior of a machine or system. It uses sensors to monitor the system's performance and compares that to a desired or reference state, then sends signals to adjust or correct the system's behavior as needed. For example, a thermostat in a heating system is a type of controller that regulates the temperature by turning the furnace on or off based on the temperature it senses.
In the context of computing, a controller can refer to software or hardware that manages or directs the operation of other components. For example, a disk controller is a hardware component that manages the flow of data between a computer's CPU and its hard disk drive, while a network controller is a device that manages communication between a computer and a network.
In gaming, a controller is an input device used to control a video game console or computer game. It typically includes buttons, joysticks, and other input mechanisms that allow the user to interact with the game.
In short, a controller is a device or system that manages, directs, or regulates the behavior of another device or system.