Plain English Meaning
A set of three cockpit instruments — airspeed indicator, altimeter, and vertical speed indicator — that work by measuring outside air pressure. They use a forward-facing tube and small flush holes on the side of the aircraft to sense pressure changes as the aircraft moves and climbs or descends.
Full Definition
The group of flight instruments that operate by sensing and comparing air pressures captured by the aircraft's pitot tube and static ports. This group includes the airspeed indicator, altimeter, and vertical speed indicator. The pitot tube measures ram (impact) air pressure from the aircraft's forward motion, while the static ports measure the ambient (still) air pressure of the surrounding atmosphere. Each instrument uses one or both of these pressure sources to display a different aspect of the aircraft's motion through the air.