What does BMX stand for?

BMX began in the early 1970s, when children began racing their bicycles on dirt tracks in southern California, inspired by the motocross stars of the time. Though originally denoting a bicycle intended for BMX Racing, the term "BMX bike" is now used to encompass race bikes, as well as those used for the dirt, vert, park, street, flatland and BMX freestyle disciplines of BMX.