sphincter

https://doi.org/10.1351/goldbook.11247
Ringlike muscle that maintains closure of a body passage or orifice by contracting, and opens it by relaxing.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 807 (https://doi.org/10.1515/pac-2015-1202)