sphincter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A ringlike muscle that normally maintains constriction of a body passage or orifice and that relaxes as required by normal physiological functioning.

from The Century Dictionary.

• noun An orbicular, circular, or annular muscle surrounding and capable of closing a natural orifice or passage of the body.
• noun The sphincter ani proper, sphincter externus, or external sphincter is a thin, flat plane of voluntary muscular fibers supplied by hemorrhoidal branches of nerves from the sacral plexus, surrounding the anus, subcutaneous and intimately adherent to the integument, of elliptical form 3 or 4 inches in long diameter, and an inch wide across. It arises from the tip of the coccyx, and is inserted into the tendinous raphe of the perineum. Like most sphincters. it consists of symmetrical lateral halves united by a raphe in front of and behind the opening it incloses.
• noun The sphincter recti, sphincter internus, or internal sphincter surrounds the lower end of the rectum, forming a muscular ring about an inch in extent and a quarter of an inch thick, and consists of an aggregation and thickening of the circular fibers of the gut. This sphincter is involuntary, and in health maintains its tonic contractility, which yields by reflex action to the pressure of the contents of the bowel.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Anat.) Of, pertaining to, or designating, a sphincter.
• noun (Anat.) A muscle which surrounds, and by its contraction tends to close, a natural opening.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy A ringlike band of muscle that surrounds a bodily opening, constricting and relaxing as required for normal physiological functioning.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a ring of muscle that contracts to close an opening