Most probably 'air craft' - a generic term which includes an extremely broad spectrum from 'manned' and 'unmanned' to sub-sonic, super-sonic, hyper-sonic, and 'extra-terrestrial'... A nice way to include a safe 'UAP' (Unidentified Aerial Phenomenon) instead of 'UFO' which has come to mean 'extra-terrestrial'.
See number 55.
It's a bull shit term. Example; Could an, "Unidentified Aerial Phenomenon" possibly turn out to be extra terrestrial in origin? The obvious answer is yes.