Sheet is defined as any material or piece of uniform thickness and of considerable length and width as compared to its thickness. As regards to metals, such pieces under 6.5 mm (1/4 in.) thick are called sheets.
See also:
"Sheet" is in the NAICS Code "32611"
(Plastics Packaging Materials and Unlaminated Film and Sheet ManufacturingPlastics Packaging Materials and Unlaminated Film and Sheet Manufacturing)
"Sheet" is in the CPC Code "41252"
Sheet piling, of iron or steel; welded angles, shapes and sections, of iron or steel