An involuntary spasm in members of the male sex, brought on by the sound of Led Zeppelin albums. The practice of air guitar often happens in front of mirrors, and frequently with an accompanying tennis racquet (though cricket bats are generally preferred for bass riffs). Long hair adds considerable impetus to the performance of air guitar, and the entire industry of Heavy Metal has grown from its suitability for stadium-wide air guitar apoplexy.
Being caught in the act of playing air guitar is considered embarrassing, probably because there is no explanation for the actions involved that would not make things considerably worse.
Companion ailments include the air drum roll, which has to be executed with finesse, especially in a crowd; the air saxophone solo, which went through a brief period of acceptability in the early 1980s before being deemed embarrassing along with 'Careless Whisper'; the air keyboard, which lost popularity after a disastrous attempt to combine the keyboard and the guitar resulted in an instrument that let keyboard players get out from behind their instruments and to the front of the stage, leaving fans with images they'd rather forget; and finally the air Walkman, in which headphone-clad sufferers find themselves accidentally playing the introduction to 'Stairway to Heaven' in front of complete strangers on the London Underground, none of whom can hear the track.
This is related to, but not the same as, singing along loudly to your car stereo in a traffic jam.