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.
Plain English
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.
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' (water), meaning 'without liquid.' The name distinguishes these instruments from earlier mercury-based barometers; an aneroid measures pressure mechanically using a sealed capsule instead of a column of liquid.
Why Pilots Care
The capsules provide the pressure reference that drives altitude indications, so any malfunction directly affects the pilot's ability to maintain assigned altitudes and clear terrain.
Analogy
Think of a small, flat tin with the air pumped out. Squeeze the outside air harder and the tin flexes inward; ease off and it springs back out. That gentle in-and-out motion is what the instrument reads.
Intuition Check
Do not read “capsule” as a pill or container for medicine here. In this context, it means a small sealed metal chamber that moves when pressure changes.
Example Sentence 1
As the aircraft climbs and outside pressure drops, the aneroid capsules in the altimeter expand, driving the altitude needle upward.
Example Sentence 2
During the preflight check the pilot verifies that the aneroid capsules are responding by comparing the indicated altitude to the known field elevation.