Definition
A completely airtight seal that prevents any exchange of gas or moisture between the inside of a sealed component and the outside environment.
Plain English
A perfect airtight closure. Nothing — no air, no moisture, no vapor — can get in or out.
Context Anchor
Seen in maintenance descriptions for sealed aircraft instruments, sensors, electronic units, and electrical feed-throughs where moisture or air leakage would damage the part.
Derivation
From Hermes Trismegistus, a figure associated with early alchemy. Alchemists were credited with a method of sealing glass tubes airtight, and 'hermetic' came to mean 'completely sealed against air.' Knowing this helps anchor the word: hermetic always implies a true airtight closure, not just a tight fit.
Why Pilots Care
Protects precision instruments from contamination that could cause inaccurate readings or failure.
Intuition Check
A hermetic seal is not just a tight cover or a good gasket. It means the part is sealed against air and moisture passing through under normal use.
Example Sentence 1
The pressure-sensing instrument is hermetically sealed, so any sign of internal moisture means the seal has failed.
Example Sentence 2
Moisture intrusion through a compromised hermetic seal can lead to instrument fogging.