I'd slightly modify your definition, Elaine, to say that it was when one partner had both feet of the ground and their weight partly (or wholly) supported by the other.
I wouldn't count simply jumping in the air as an airstep as such.
Height limits are simply imposed by competition organisers - an aerial can go
right over the head of the supporting partner in principal.
For Justine:
airsteps and aerials are the same thing.
To be honest my feeling is that they have no place in social dancing. The dangers of injury to one or both of the participants are extremely high and it seems silly to take that risk for the most part.
In any case done badly (and you have to be pretty damn good to do airsteps well) they look ungainly and you don't see many amateur dancers carry them off well much less to say stay in time with the music in the process.
There are one or two guy's about who insist on trying aerials with anyone they can persuade to dance with them. I suppose they think it impresses. It doesn't do anything for me.
All in my opinion, of course.
Andy