A blazed grating is a special type of diffraction grating.
Blazed gratings produce maximum efficiency at a specified wavelength; that is, a diffraction grating that is "blazed at 250nm" will operate most efficiently when light with a wavelength of 250 nm passes through the grating.
Like standard diffraction gratings, blazed gratings diffract incoming light using a series of grooves. However, in blazed gratings the grooves have been manufactured such that they form right angles with a specified "blaze angle," which is the angular distance from the surface normal of the diffraction plate. The magnitude of the blaze angle determines the wavelength at which the grating will be most efficient.
Blazed gratings are useful in planet-finding astronomy, and are used on the successful HARPS echelle spectrograph.