ILD = (Indention Load Deflection):

In general, ILD is used to define the level of firmness or softness of a piece of foam (or load bearing characteristics). ILD is defined by how much weight/lbs. it takes to compress a sample of foam by 25% (or to 75% of its original height or thickness). Example: If it takes 15lbs of load/force to compress the foam sample by 25%, the foam has a 15ILD rating. The higher the ILD the harder the foam.