Intake air
From Kerbal Space Program Wiki
Intake air is a resource that fuels jet engines. It is provided by air intakes running in a suitable atmosphere. Stock parts are unable to effectively store air; it will leak out of opened intakes, and closed ones hold too little (0.2 to 1.0 units) to be useful. "Fuel" tanks can be created that hold intake Intake Air, in which case it behaves much like LiquidFuel or Oxidizer.