“Mil” and “thou” are the same. They are imperial measurements both are synonyms for 0.001 inches.  This unit is normally referred to as a “thou” (which is short for a thousandth), or (particularly in the United States) a mil. Mil has its origins in the metric prefix “milli”, which is Latin for “one-thousandths”.  The plural of a mil is mils and the plural of thou is thou.