A failure of equipment to operate properly due to electrons being unable to leap through the atmosphere from a male plug to a female connector. "Was the reason that fixture wouldn't come on an air-gap?" "Yeah, it accidentally got unplugged." This entry inspired by http://www.controlbooth.com/forums/glossarys/9395-headphone-short.html. Not to be confused with spark-gap or arc-gap, which are real, actual things.