a thou, a mil or point, a "milli-inch"

thou

A thou, also known as a mil or point, is a unit of length equal to 0.001 inch (a "milli-inch" or a "thousandth of one inch"). It is sometimes used in engineering and in the specification of:

the thickness of items such as paper, film, foil, wires, paint coatings, latex gloves, plastic sheeting, and fibers; and

manufacturing dimensions and tolerances.

in the manufacture of certain racing engines [Normally the space between the valves and pistons, or camshaft and valves]

In the United States, as the metric system became more common thou began to replace mil among technical users due to the possible confusion with millimeters. However, the mil is still in common use in the United States for the thickness of plastic sheeting or bags.... (© Wikipedia)