The most general definition of foam is a substance that is formed by trapping many gas bubbles in a liquid or solid. It can also refer to anything that is analogous to such a phenomenon, such as quantum foam. Often people mean polyurethane foam (foam rubber), Styrofoam or some other manufactured foam when they are using the term.

From the early 20th century, various types of specially manufactured solid foams came into use. The low density of these foams made them excellent as insulators and flotation devices. And their lightness and compressibility made them ideal as packing materials and stuffings. Some liquid foams also found uses in extinguishing fires, especially oil fires .

Foam, in this case meaning "bubbly liquid", is also produced as an often unwanted by-product in the manufacture of various substances. For example, foam is a serious problem in the chemical industry, especially for biochemical processes. Many biological substances, for example proteins, easily create foam on agitation and/or aeration. Foam is a problem because it alters the liquid flow and blocks oxygen transfer from air (therefore preventing microbial respiration in fermentation processes). For this reason, antifoam compounds, like silicone oils, are added to prevent these problems.

Foaming around the mouth can be a symptom of rabies in animals. The term sea foam is used to describe the foam that forms on top of seawater from the action of waves. In some ways, leavened bread is a foam, as the yeast causes the bread to rise by producing tiny bubbles of gas in the dough.

Types of manfucatured foam

• acoustic foam
• anti-static foam
• closed cell foam - a firm type of foam
  • Aerogel the world's lowest density solid
  • Airex foam or FloTex foam
  • Latex foam
  • polychloroprene foam
    • Neoprene
  • polyethylene foam
    • EVA foam or polyethylene vinyl acetate foam
  • polyisocyanurate foam
  • polystyrene foam
    • Styrofoam
  • polythene foam
  • Syntactic foam, with extremely high compressive strength.
• fire retardant foam or CMHR (Combustion Modified High Resillient) foam
• open cell foam - a more breathable and flexible foam
  • polyurethane foam
    • memory foam
• spray foam