The most general definition of foam's a substance that's formed by trapping many gas bubbles in a liquid or solid. It can also refer to anything that's analogous to such a phenomenon, such as quantum foam. Often the term's used in reference to polyurethane foam (foam rubber), XPS foam, Polystyrene, or many other manufactured foams. It can be considered a type of colloid. From the early 20th century, various types of specially manufactured solid foams came into use. The low density of these foam… (More on Foam)