In a Michelson interferometer or other two-beam interferometer, the position of the moving mirror at which the optical paths are equal.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 679 (https://doi.org/10.1515/pac-2019-0203)