Meaning of etalon

e•ta•lon

Pronunciation: (āt'l-on"), [key]
— n. Optics.
  1. an interferometer consisting of two glass plates that reflect approximately half of each ray of light incident upon them and that are separated by a small, fixed distance: used to compare wavelengths and to study atomic spectra.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.