ECU Engine Control Unit. Your car's computer. It's the heart of the engine management system in a modern car, collecting many inputs from sensors around the car and controlling all functions of the engine such as fuel injection and heat management.
ECU is a type of electronic control unit that determines the amount of fuel, ignition timing and other parameters an internal combustion engine needs to keep running. It does this by reading values from multidimensional performance maps, using input values (e.g. engine speed) calculated from signals coming from sensor devices monitoring the engine.
Before ECU's, fuel injection, ignition timing, and idle speed were directly controlled by mechanical and pneumatic sensors and actuators.