air gap

Short definition from Wiktionary

Noun

1. 1
   A network security measure that consists of ensuring that a secure computer network is physically isolated from unsecured networks, such as the public Internet or an unsecured local area network.
2. 2
   A gap where there is a vacuum (not actually air), used to reduce power consumption in microelectronics.
3. 3
   A type of spark gap using a tube filled with air (which is mostly nitrogen), nitrogen alone, or other gases.