Plain English Meaning
A small sealed metal can with the air mostly removed. When outside air pressure changes, the can flexes, and that flexing is what drives the needle on the instrument.
Full Definition
A sealed, flexible metal capsule (or stack of capsules) with most of the air evacuated, used as the pressure-sensing element in instruments such as altimeters, vertical speed indicators, and barometers. As outside air pressure changes, the capsule expands or contracts, and that mechanical movement is converted into an instrument reading.
Context Anchor
Seen in aircraft instrument discussions, especially altimeters, pressure instruments, and airframe maintenance descriptions.
Derivation
From Greek 'a-' meaning 'without' and 'neros' meaning 'liquid' — literally 'without liquid.' Older barometers used a column of mercury (a liquid) to measure pressure. An aneroid does the same job without any liquid, using a flexing sealed capsule instead.