1) A framework or latticework having identical parallel rods that are evenly spaced. 2) In optics, grating is any repetitive structure that can separate the light into its constituent wavelengths. Commonplace CDs and DVDs have fine grooves carved on them, which creates a reflective grating. Physicists and astronomers often use diffraction gratings to determine the wavelengths composing the light being viewed.
To learn more about gratings click the link to the Solid-State Lighting Pamphlet.
(n.) - c.1400, from M.L. grata "lattice," from L. cratis "wickerwork."