Plain English Meaning
A small computer that takes pressure and temperature readings from sensors on the outside of the aircraft, does the math, and feeds your speed, altitude, and climb-rate information to the instruments.
Full Definition
An electronic device in modern aircraft that gathers raw air-pressure and temperature inputs from the pitot tube, static ports, and outside air temperature probe, then calculates and outputs flight parameters such as airspeed, altitude, vertical speed, Mach number, and true air temperature to the cockpit displays and other .