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