A grating is any regularly spaced collection of essentially identical, parallel, elongated elements. Gratings usually consist of a single set of elongated elements, but can consist of two sets, in which case the second set is usually perpendicular to the first (as illustrated). When the two sets are perpendicular, this is also known as a mesh.
Read more about Grating: Grating As Bridge Decks, Gratings As Filters, Diffraction Gratings, Gratings As Pictures