Memory foam was originally developed by NASA in the 1960s as a way to improve the safety and comfort of aircraft cushions. It is now a commercially available product which is particularly popular in mattresses.
Memory foam's defining feature is its ability to soften and mold to a warm body rather quickly and then gradually return to its original shape.
For example, if you press your hand into memory foam, you will likely see a hand shaped indentation for several seconds afterwards.
Memory foam is polyurethane which has had chemicals added to it which increase its viscosity and density. The term "viscoelastic" is often used to describe memory foam.
Newer generations of memory foam have an open-cell structure which improves breathability and temperature regulation.
As with other polyurethane products, memory foam can be combustable and therefore when used in bedding it will have been treated with chemicals to make it fire retardant.
It is not recommended to let babies and young children sleep on memory foam as they may find it difficult to turn over and run the risk of suffocation.
Memory foam mattresses are usually denser than other foam mattresses, which makes it more supportive but also heavier.
Different densities of memory foam are available, less dense memory foam will feel softer but repeated compression and decompression will shorten it's life.
Higher density memory foam will feel firmer but have greater longevity.