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A hermetic seal is an airtight seal. For example, tin cans are hermetically sealed. The term is often used to describe electronic parts that are designed and intended to secure against the entry of microorganisms and to maintain the safety and quality of their contents. Applications include semiconductor electronics, thermostats, optical devices, and switches. The food, chemical, and medical industries all have applications for the use of such "airtight" packaging, such as induction sealing, as well. High-end coffins, too, are often made to be "hermetically sealed" and must be of metal or of other material with metal lining, and constructed so that when closed and fastened the coffin is completely airtight. In some nuclear reactor designs, the reactor is housed in a hermetically sealed reactor vessel.

The expression "hermetically sealed" finds its roots in Hermes Trismegistus, a syncretism of the Greek god Hermes and the Egyptian god of wisdom, Thoth. Its origin can be traced back to about 300 AD. Hermes Trismegistus purportedly authored several books containing secrets of alchemy and mystic philosophy, the Hermetica. In the 17th century, English writers began using the adjective hermetic to refer to things that were sealed or secret. An early "hermetically sealed" container featured in the dramatic demonstration of the force of air pressure in creating a hermetic seal in 1663, when for the enlightenment and entertainment of the court of Friedrich Wilhelm I of Brandenburg, Otto von Guericke joined two copper hemispheres (Magdeburg hemispheres) and pumped the air out of the enclosure. Then he harnessed a team of eight horses to each hemisphere and showed that they were not able to separate them.

The U.S. Constitution, U.S. Declaration of Independence and U.S. Bill of Rights are hermetically sealed with helium gas in glass cases housed in the U.S. National Archives in Washington, DC.

The word hermetic comes from the syncretism of the Greek god Hermes and the Egyptian Thoth; this figure was also a mythological alchemist known as Hermes Trismegistus. The latter has three books attributed to him, the Emerald Tablet, the Corpus Hermeticum and The Kybalion. He was believed to possess a magic ability to seal treasure chests so that nothing could access their contents. Alchemists also frequently used distillation in their experiments, and needed an airtight seal to improve the efficiency of their alembic stills. Most alchemists, though, were considered to be Hermetics for adopting the philosophy of the Emerald Tablet or the Corpus Hermeticum.