noun
airtightness; hermeticity
Technical term used in engineering, construction, and product design to describe the degree to which a container, seal, or structure prevents air leakage.
この容器は気密性が高いので、食品が長持ちします。
This container has high airtightness, so food stays fresh longer.
建物の気密性を測定する試験が行われた
A test was conducted to measure the building's airtightness.
密閉 means 'sealing tightly' (action/state), whereas 気密性 is the resulting quality of being airtight.
Compound of 気密 (airtight) + 性 (property, -ness). The word is a modern technical coinage.