Etalon

Part of speech: noun

Definitions

1. A precise optical instrument consisting of two partially reflective surfaces used to measure wavelengths by interference
2. An optical tool that creates interference patterns to accurately gauge the wavelength of light
3. A device with parallel reflective surfaces that produces interference fringes to determine light wavelength

Etymology: The French "étalon" meant a standard measure, the reference against which other measures were checked. In English, "etalon" is a borrowing from that French sense, and it kept the same core idea: something whose exact dimensions define what everything else must match. The instrument named here applies that idea to light. It splits and recombines beams so that the bright and dark bands they produce reveal wavelengths with great precision, turning the reference-standard sense of the French word into a tool for measuring the properties of light itself.