Intake air is a resource required by jet engines. It is provided by air intakes while in an oxygenated atmosphere. The amount of air taken in varies proportionally with speed and inversely with altitude. On Kerbin, air becomes too thin for jet operation at roughly 30–40 km, depending on speed and a craft's ratio of intakes to engines.
Stock parts are unable to effectively store intake air; it will leak out of opened intakes, and closed intakes hold too little (0.2 to 1.0 units) to be useful.