A device used to break light into its component wavelengths. It is usually composed of a material with tiny grooves cut into it. These disperse the light as it passes through or bounces off the grating (depending on the type of grating). Physicists and astronomers often use diffraction gratings to determine the wavelengths composing the light being viewed.
c.1374, from L. diffusionem, from stem of diffundere "scatter, pour out," from dif- "apart, in every direction" + fundere "pour."