Plain English Meaning

A small sealed metal bellows with the air sucked out. When outside air pressure changes, the bellows flexes — and that tiny movement is what makes pressure instruments work, without needing any liquid inside.

Full Definition

A sealed, flexible metal capsule (or stack of capsules) with most of the air evacuated from inside, used as the pressure-sensing element in instruments such as the altimeter, vertical speed indicator, and some airspeed indicators. As the surrounding atmospheric pressure changes, the capsule expands or contracts, and that movement is transferred through mechanical linkages to drive the instrument's pointer.

Context Anchor

Seen in discussions of altimeters and other aircraft instruments that use outside air pressure to give a reading.