back2fixie's answer is good; there is not so much a hard and fast definition of "track bike" as there are design features that make a bike more and more "track specific". to the previous list i would add rear-facing "track ends" instead of dropouts, fork and seatstay bridge not drilled for brakes, circular cross-section fork blades instead of elliptical (for steel anyway). 1/8" drive train instead of 3/32". anything else?