Hermetic describes a type of closure that is complete and airtight. Reed Switches are hermetically sealed and this prevents the switching area from effects of the atmosphere, thereby making them safe to use in explosive environments. Reed Switches are one of the few devices which continue to be used in fuel tanks for accurate measurement of volume of fuel.
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 rubber and plastics. Hermetic seals are essential to the correct and safe functionality of many electronic and healthcare products. Used technically, it is stated in conjunction with a specific test method and conditions of use.