An Air Check is the stereo version of a K.K. Slider song, often soundingextremely different than the version sung by the musician himself. The Air Check keeps the same melody, but because of the instrumental differences, the two versions are drastically unalike.
Animal Crossing is the only game where Air Checks are known as "Air Checks". In the two succeeding games, they are called "Bootlegs", and "Boots" in New Leaf.