The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sphincter \Sphinc"ter\, a. (Anat.)
   Of, pertaining to, or designating, a sphincter; as, a
   sphincter muscle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sphincter
    n 1: a ring of muscle that contracts to close an opening [syn:
         sphincter, anatomical sphincter, sphincter muscle]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "sphincter":
   O, annular muscle, annulus, areola, aureole, chaplet, circle,
   circuit, circumference, circus, closed circle, corona, coronet,
   crown, cycle, diadem, discus, disk, eternal return, fairy ring,
   garland, glory, halo, lasso, logical circle, loop, looplet,
   magic circle, noose, orbit, radius, ring, rondelle, round, roundel,
   saucer, vicious circle, wheel, wreath