zero path difference 

https://doi.org/10.1351/goldbook.08305
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 [Terms] [Paper]