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 [Terms] [Paper]