1. Completely sealed, especially against the escape or entry of air: Identified by Neoplatonists, mystics, and alchemists with the Egyptian god Thoth as "Hermes Trismegistos", "Thrice-Great Hermes", is credited with inventing the process of making a glass tube airtight (a process in alchemy) using a secret seal; and from his name the adjective hermetic, meaning "airtight", was derived.

There are hermetic materials that are made airtight by means of fusion or sealing.

2. Impervious to outside interferences or protected from outside influences: The hermetic confines of an isolated life may lead to misery.

The religious group lived in a hermetic society.

3. Having to do with the occult sciences, especially alchemy; magical; alchemic; obscure; difficult to understand: In the seventeenth century, English hermetic meant "pertaining to alchemy" and the occult sciences in general. Alchemy, and later chemistry, was itself known as the hermetic art, philosophy, or science.
4. Etymology: from New Latin, the adjective hermeticus was formed from the name of the god Hermes Trismegistos, and the English borrowed this term from New Latin."

An engine that is used to operate a refrigerator: A compressor driven by a hermetic motor has the advantage of the compressor shaft not having to pass through a seal between the outer environment and the refrigerant medium inside the chiller.

A device that is used to close something so it can not be penetrated by air or fluids: The engineer put a hermetic seal on the container so no air could get into it.