A type of optical interferometer in which light is split into two beams whose paths can be widely separated before they are recombined at another beam splitter, typically used to measure large-scale variations in refractive index.
1950s; earliest use found in Max Born (1882–1970), physicist. From the names of Ludwig Mach, Austrian scientist, and Ludwig Zehnder, Swiss scientist, each of whom independently invented an instrument of this type in 1891 (documented respectively in Zeitschr. f. Instrumentenkunde 12 89, and Zeitschr. f. Instrumentenkunde 11 275) + interferometer.