The most general definition of foam is a substance that's formed by trapping many gas bubbles in a liquid or solid. A foam is normally an extremely complex system consisting of polydisperse gas bubbles separated by draining films]].. The term foam may also refer to anything that's analogous to such a phenomenon, such as quantum foam, polyurethane foam (foam rubber), XPS foam, Polystyrene, or many other manufactured foams. Fine foam can be considered a type of colloid. Structure of foams Real-li… (More on Foam)