The Collaborative International Dictionary of English v.0.48:

Sprocket \Sprock"et\ (spr[o^]k"[e^]t), 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.
       [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

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, sprocket wheel]
    3: tooth on the rim of gear wheel [syn: cog, sprocket]