etalon

noun

eta·lon

ˈātᵊlˌän,

ˈet-

plural -s

: an interferometer in which two parallel partially silvered glass plates at a fixed distance apart produce by multiple reflection interference spectra of high dispersion and resolution adapted to the fine-structure analysis of spectrum lines

Word History

Etymology

French étalon standard of weights and measures, from Middle French estalon, estelon, probably of Germanic origin; akin to Frisian stal shape, form