The thinning hair atop a balding man's head that you can see right through.
In the last seasons of his classic sitcom, Jerry Seinfeld's air hair became quite noticeable. So was Nicholas Cage's - before he really went bald and had to wear a piece in his films.
(This term was first coined by Mark Hänser of Boston, MA)
In the same vein as the term 'bed head', what rather unstylistically results from exposing a head of hair to the open air whilst cycling without the use of a helmet, i.e. a random rearrangement of hair strands.
'Boris doesn't wear a helmet y'know'
'Yes and he pays the price by getting air hair'
'He must cycle a lot then'
'What makes you say that?'
'He always has air hair'