Referring to the opening of the vagina that usually occurs at sexual maturity and is maintained by hormonal influences pre-menopausally.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 818 (https://doi.org/10.1515/pac-2015-1202)