Plain English Meaning

Small sealed metal discs with the air sucked out of them. They get bigger when outside air pressure drops and smaller when it rises, and that tiny movement drives the needle on a pressure-based instrument like the altimeter.

Full Definition

Sealed, flexible metal wafers from which most of the air has been evacuated, used as the sensing element in altimeters and other pressure instruments. They expand as the surrounding air pressure decreases and contract as it increases, and this physical movement is mechanically linked to the instrument's display.

Context Anchor

Seen in discussions of how pressure instruments, especially the altimeter and vertical speed indicator, turn air pressure changes into cockpit indications.

Derivation

Aneroid comes from the Greek 'a-' (without) and 'neros' (), meaning 'without liquid.' The name distinguishes these instruments from earlier -based barometers; an aneroid measures pressure mechanically using a sealed instead of a column of liquid.