"SPOT AVAILABILITY":

In general, the availability of a benefit (for example, a seat in an airplane) without any advance notice or commitment by either the party who owns or controls the benefit, or the party benefitted.

Specifically applied to hitchhiking, the availability of a ride as the vehicle is proceeding to its destination -- or, more precisely, at the last moment when boarding or entering the vehicle is logically possible. And, specifically applied to AIRhitching, the availability of a ride as the aircraft is about to depart from its current airport, to travel to its airport of destination.