Source : Webster's Revised Unabridged Dictionary (1913)

Sphincter \Sphinc"ter\, n. [NL., fr. ??? to bind tight.] (Anat.)
   A muscle which surrounds, and by its contraction tends to
   close, a natural opening; as, the sphincter of the bladder.

Sphincter \Sphinc"ter\, a. (Anat.)
   Of, pertaining to, or designating, a sphincter; as, a
   sphincter muscle.

Source : WordNet®

sphincter
     n : a ring of muscle that contracts to close an opening [syn: {anatomical
         sphincter}, {sphincter muscle}]