From Longman Dictionary of Contemporary Englishsphinctersphinc‧ter /ˈsfɪŋktə $ -ər/ noun [countable] medical HBHa muscle that surrounds an opening in your body, and can become tight in order to close the opening
Examples from the Corpus
sphincter• the anal sphincter• The sphincters become leaky, and the urine overflows.• Thus it should not be assumed that older patients with ulcerative colitis have weaker sphincters than younger patients.Origin sphincter (1500-1600) Late Latin Greek, from sphingein “to tie tightly” 