A US half-sheet pan is typically about 18 by 13 inches, measured at its outer edges, with a shallow rim. It is used for cookies, roasting vegetables and other oven tasks. Interior usable area is smaller than the nominal dimensions.
It is half the footprint of a commercial full sheet and twice that of a quarter sheet. Check actual dimensions before buying fitted racks, lids or reusable liners; manufacturers and regional conventions vary.
Reference: Further reading.