Plain English Meaning

Small sealed metal discs with almost no air inside. When the air around them gets thinner, they puff out a little; when the air gets denser, they squeeze in. That tiny movement is what makes the altimeter needle turn.

Full Definition

Sealed, flexible metal capsules inside a pressure-sensitive instrument from which most of the air has been evacuated. They expand as the surrounding air pressure decreases and contract as it increases, providing the mechanical movement that drives the indication on instruments such as the altimeter and vertical speed indicator.

Context Anchor

Seen in discussions and diagrams of how a pressure altimeter works.

Derivation

Aneroid comes from the Greek 'a-' meaning 'without' and 'neros' meaning 'liquid' or 'water.' It literally means 'without liquid,' distinguishing this kind of pressure from older ones that used or other . A simply describes the , flat, disc-like shape of the .