What is "monochromator"

n. (context physics English) An optical device, consisting of one or more slits, that selects a narrow band of wavelengths from a broader spectrum.

A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is from the Greek roots mono-, "single", and chroma, "colour", and the Latin suffix -ator, denoting an agent.