Stands for Indentation Load Deflection. Usually used with latex as a measure of firmness but has been replaced with IFD in other foam types. It indicates the amount of force it takes to compress a layer of foam by specific 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 by 25% of its height. There are also versions however that use 6" of material (most latex for example) or a greater or lesser percentage of compression (many European or Asian manufacturers use 40% instead of 25%) all of which give different results so ILD ratings are not always comparable between different materials.