The amount of weight it takes to compress a layer of material a certain percentage of its depth. The most common form uses a 50 sq in deflector foot to depress a 20″ x 20″ layer of material that is 4″ thick down 25% of its height. This is used almost universally for polyfoam around the world. There are also versions however that use 6″ of material (some Talalay latex) or a greater or lesser percentage of compression (many European or Asian manufacturers) all of which give different results and can add to the information about the material.