The term 'Unit of Thickness' can be defined as a measure of the thickness of a material. Two common units of thickness are MIL and PLY. MIL stands for 'thousandths of an inch' and is a unit of measurement used to measure the thickness of plastic films, paper, and other thin materials. PLY stands for 'plywood' and is a unit of measurement used to measure the thickness of wood and other thicker materials.