‘括约肌’ (kuò yuē jī) is a medical term referring to a circular muscle that normally maintains constriction of a natural passage or sphincter and which can relax as needed to allow passage of substances—such as food, urine, or feces. The three characters combine meaningfully: ‘括’ (kuò) means ‘to encircle’ or ‘to constrict’, ‘约’ (yuē) means ‘to restrain’ or ‘to regulate’, and ‘肌’ (jī) means ‘muscle’. Together, they literally describe a muscle whose function is to constrict and control opening and closing.

This term appears frequently in anatomy, physiology, and clinical contexts—especially when discussing digestive, urinary, or reproductive systems. Common examples include the anal sphincter (肛门括约肌), urethral sphincter (尿道括约肌), and cardiac sphincter (贲门括约肌). It is a formal, technical word used primarily by healthcare professionals, students, and in scientific texts—not in everyday conversation.