A hermetic seal is any type of sealing that makes a given object airtight (preventing the passage of air, oxygen, or other gases). The term originally applied to airtight glass containers, but as technology advanced, it applied to a larger category of materials, including metals, and rubber. Hermetic seals are essential to the correct and safe functionality of many electronic and healthcare products.

Used technically, the term is stated in conjunction with a specific test method and conditions of use. Colloquially, the exact requirements of such a seal varies with the application.

The word hermetic is indirectly related to the Greek god Hermes. The hermetic seal comes from alchemy in the tradition of Hermeticism, which was named after Hermes Trismegistus, a legendary figure who, according to the Hermeticists, invented the process of making a glass tube airtight using a secret seal.

Some kinds of packaging must maintain a seal against the flow of gases, for example, packaging for some foods, pharmaceuticals, chemicals, and consumer goods. The term can describe the result of some food preservation practices, such as vacuum packing and canning. Packaging materials include glass, aluminum cans, metal foils, and gas-impermeable plastics.

Some buildings designed with sustainable architecture principles may use airtight technologies to conserve energy. Green buildings may include windows that combine triple-pane insulated glazing with argon or krypton gas to reduce thermal conductivity and increase efficiency. In landscape and exterior construction projects, airtight seals may be used to protect general services and landscape lighting electrical connections/splices. Airtight implies both waterproof and vapor-proof.

Hermetic seals enable human spaceflight by sealing breathable air in the habitable areas; without hermetic sealing, the air would expand and scatter to fill the near-vacuum of space, resulting in depressurization, which can cause injury or death in living organisms.

Airlocks, which rely on hermetic sealing technology, are used to allow things and organisms to enter and exit two areas with different pressures or compositions. Airlocks are used in underwater diving, underground construction, mining, and spaceflight.

Electronic devices that use hermetic sealing include semiconductor electronics, thermostats, optical devices, MEMS, and switches. Electrical or electronic parts may be hermetic sealed to secure against water vapor and foreign bodies to maintain proper functioning and reliability.