Wiktionary
ground proximity warning system
n. (context aeronautics English) A device which warns the pilot that the aircraft may be in danger of collision with the ground.
Wikipedia
Ground proximity warning system
A ground proximity warning system (GPWS) is a system designed to alert pilots if their aircraft is in immediate danger of flying into the ground or an obstacle. The United States Federal Aviation Administration (FAA) defines GPWS as a type of terrain awareness warning system (TAWS). More advanced systems, introduced in 1996, are known as enhanced ground proximity warning systems (EGPWS), although sometimes called terrain awareness warning systems.