From AMS Glossary
A unit of thermal insulation, usually applied to clothing or bedcovers.
It is defined as the amount of insulation necessary to maintain comfort and a mean skin temperature of 33°C (92°F) for a person who is producing heat at the standard metabolic rate (50 Kcal m-2 of body surface per hour; one met) in an indoor environment characterized by a temperature of 21°C (70°F), relative humidity of less than 50%, and air motion of 6.1 m min-1. If if is assumed that 76% of the metabolic heat is lost through the clothing, the unit can be defined in physical terms as the insulation that will restrict heat loss to 1 Kcal m-2h-1 with a temperature gradient of 0.18°C across the fabric. In the first approximation, an insulation of one clo is provided by clothing material with a total thickness of 0.64 cm and air layers (between skin and clothing and between inner and outer garments) of about 0.51 cm.
Newburgh, L. H., ed. 1949. Physiology of Heat Regulation and the Science of Clothing. pp. 297, 299, 445.
Winslow, C.-E. A., and L. P. Herrington 1949. Temperature and Human Life. 132–140.