The weight per cubic foot (in imperial measurement)of a foam. With some types of foam such as polyfoam and memory foam, density is an important part of quality. Comparing different types of foam with different densities in terms of quality is meaningless. Good Quality Memory Foam is usually over 5 lbs/cu ft and high quality Polyfoam (HR grade) is 2.5lbs/cu ft or higher.
How long a material will keep its desirable properties and used also to indicate how long a material will take to completely break down.