diffraction grating

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A polished surface, usually glass or metal, having a large number of very fine parallel grooves or slits, and used to produce optical spectra by diffraction of reflected or transmitted light.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics a surface having a large number of closely spaced lines or slits; used to produce optical spectra by mutual interference

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun optical device consisting of a surface with many parallel grooves in it; disperses a beam of light (or other electromagnetic radiation) into its wavelengths to produce its spectrum