Cycling, also called bicycling or biking, is the use of bicycles for transport, recreation, exercise or sport.[1] People engaged in cycling are referred to as "cyclists",[2] "bicyclists",[3] or "bikers".[4] Apart from two-wheeled bicycles, "cycling" also includes the riding of unicycles, tricycles, quadracycles, recumbent and similar human-powered vehicles (HPVs). Bicycles were introduced in the 19th century and now number approximately one billion worldwide.[5] They are the principal means of transportation in many parts of the world. Cycling is widely regarded as a very effective and efficient mode of transportation[6][7] optimal for short to moderate distances.