Definition
Sealed completely so that no air, gas, or moisture can pass in or out. A hermetically sealed unit is closed off from the surrounding atmosphere, typically by fusing or welding the housing rather than using removable fasteners or gaskets.
Plain English
Closed up tight enough that nothing — no air, no moisture, no gas — can get in or out.
Context Anchor
Seen in aircraft maintenance descriptions of sealed engine, instrument, electrical, or pressure-related components.
Derivation
From Hermes Trismegistus, a figure linked to early alchemy, whose followers were said to seal vessels so completely that nothing could escape. The term carried into modern use to describe airtight, fully closed seals.
Why Pilots Care
Protects sensitive equipment such as vacuum-driven instruments and electronic units from moisture, dust, and pressure changes that could cause failure.
Intuition Check
Do not read hermetically sealed as just “closed.” A closed cover may still leak; a hermetically sealed part is intended to block air and moisture from passing through the seal.
Example Sentence 1
The aneroid capsule inside the altimeter is hermetically sealed so changes in outside air pressure act only on its outer surface.
Example Sentence 2
A hermetically sealed relay cannot be opened and must be replaced as a complete unit.