(two parallel reflective surfaces separating light according to wavelength)
An etalon is an optical device with two
partially-reflective surfaces spaced a specific distance apart
that cancels EMR at some wavelengths and
strengthens it at others. It can consist of partially-transparent
mirrors with an air or vacuum gap between them,
or the two surfaces of a single transparent plate.
The term is essentially another name for a Fabry-Pérot interferometer (FPI),
or is a name for an FPI's inner optics.