From late Latin sphincter, from Greek sphinkter, band, contractile muscle, from sphingein, to bind tight.

A sphincter is a ring-like muscle that normally maintains constriction of a natural body passage or orifice and that relaxes as required by normal physiological functioning.

Some examples of sphincters include:

• Anus
• Cardia, at the upper portion of the stomach
• Sphincter urethrae, or Urethral sphincter
• Pyloric sphincter, at the lower end of the stomach
• Sphincter pupillae, or Pupillary sphincter, belonging to the iris in the eye