Plain English Meaning
A small sealed metal bellows inside the airspeed instrument that senses both air pressure and air temperature. Because air gets thinner and colder with altitude, the bellows uses both readings to show the airspeed correctly for the conditions outside.
Full Definition
A sealed, flexible metal capsule used inside a true airspeed indicator that expands and contracts in response to changes in outside air pressure, with a built-in mechanism that adjusts its movement to account for changes in outside air temperature. This combined response allows the instrument to convert indicated airspeed into true airspeed automatically as the aircraft or descends through air of varying .