Source : Webster's Revised Unabridged Dictionary (1913)

Sprocket \Sprock"et\, n. [Etymology uncertain.] (Mach.)
   (a) A tooth or projection, as on the periphery of a wheel,
       shaped so as to engage with a chain.
   (b) A sprocket wheel.

Source : WordNet®

sprocket
     n 1: roller that has teeth on the rims to pull film or paper
          through
     2: thin wheel with teeth that engage with a chain [syn: {sprocket
        wheel}]
     3: tooth on the rim of gear wheel [syn: {cog}]