Why Is It Called Memory Foam?
Memory foam is a type of polyurethane foam, but because of its special chemistry it has properties that other foams don't have.
Top among these properties is that memory foam recovers slowly from compression when it is in it viscous, or firm, state.
When is memory foam viscous? That is another way in which memory foam is unique in the foam world. Memory foam is temperature sensitive, and so actually becomes firmer at colder temps and softer at warmer/more humid environments.
Each memory foam has its own range of temperatures that it turns from elastic (softer, and feels more like regular foam), to more viscous (harder -- and if you put in freezer, can get brick hard).
And this can be important for sleep comfort, since some memory foams get firmer below 70 degrees, and on these memory foams turning over can be difficult since you form a soft, cocoon around you and the surrounding foam can be firm and hard to turn into.
But I digress. Back to the memory feature of memory foam.
When memory foam gets firmer, it will take longer for it to come back to its original shape. So if you press your hand in to it when it is cold, the impression will stay for a few seconds before the foam comes back to its original state.
So the foam has a memory -- it keeps the hand shape for a bit, and so is said to have a memory of your hand's impression.
That is why this particular type of foam became known as memory foam.
In the trade, it is more accurately known as visco-elastic foam because of its viscous and elastic natures at different temps.