a bicycle with only one rear cog or "gear" which is fixed directly onto the hub of the wheel. This style of bicycle preceeded the more modern freewheel single speed which allowed the rider to stop pedaling whilst coasting. Still more modern yet, is the multi-speed freewheel or freehub design which is just an expansion upon the single speed freewheel.
The fixed gear was most popularly used as track or "velodrome" racing machine but has since made it to the streets and has seen a rise in outdoor road usage. Currently, in many large cities across the world, the fixed gear, or "fixie" as it so often called, has become the focal point of a growing urban subculture which includes bike messengers and the like. This subculture is seen as being quite countercultual even in the cycling community and is often associated with the punk rock/anarchist scene.
Eddie Merckx, a Belgian cyclist, is revered as one of the greatest fixed gear racers in history.