a device that controls the operation of electrical equipment (such as air conditioning units and lights) according to preset time and schedules.

A solid-state control system that continually scans its user program. The controller has a user-programmable memory for storage of instructions to implement specific functions such as I/O control, logic, timing, counting, report generation, communications, arithmetic, and data file manipulation. A controller consists of a central processor unit, input/output interface, and memory. A controller is designed as an industrial control system. The program is generally in the form of ladder logic.

a small industrial strength computer used to control real world actions, based on its program and real world sensors

a tad more complex than a simpler controller with fixed functionality

Sequence controller of stored program system.