Plain English Meaning
Instruments in the cockpit that work by measuring pressure. Some measure outside air pressure to tell the pilot things like altitude and speed. Others measure pressure inside the engine or systems to show whether everything is running correctly.
Full Definition
A category of aircraft instruments that measure air pressure -- either from the surrounding atmosphere, from inside the engine, or from various aircraft systems -- and display that information to the pilot. This group includes pitot-static instruments (altimeter, airspeed indicator, vertical speed indicator), engine pressure gauges (manifold pressure, , ), and other pressure-driven indicators used to and health.