Hermetic is an adjective, and it is used to described something made airtight by fusion or sealing. It can also be used more broadly to indicate anything isolated from external factors. The adverb is hermetically.
Inside the somewhat hermetic Basque community here, which mixes at its own social club and at a handful of small restaurants like Bar Basque and Urrutia, which offer specialties like Txistorra sausages and Txakolí wine, the indictment has brought uncomfortable attention. (NY Times)
Israel will develop anti-artillery and tunnel-detection technology to insure that the quarantine is hermetic. (The Economist)