Definition of air gap:
(n) :
(networking, computer security) 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.
(n) :
A gap where there is a vacuum (not actually air), used to reduce power consumption in microelectronics.
(n) :
A type of spark gap using a tube filled with air (which is mostly nitrogen), nitrogen alone, or other gases.