Webster's English Dictionary

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grating

grat.ing \'gra-t-in\ n 1: a partition, covering, or frame of parallel bars 
   or crossbars : GRA TE 2: a wooden or metal lattice used to close or floor 
   any of various openings  3: a system of close equidistant and parallel 
   lines or bars ruled on a poli shed surface to produce spectra by 
   diffraction