Styrofoam is that white, very light plastic foam used for lunch boxes, cups, and the protective blocks packed around a new television. It is light because it is mostly air, and it keeps heat and cold from passing through — which is why hot soup in a Styrofoam bowl stays hot and your hand does not burn.

Here is the interesting part, and it is a good lesson about English generally. Styrofoam is a brand name, owned by a chemical company, and strictly speaking the real product is a blue building material, not the white cups at all. But almost nobody knows or cares. In everyday American English, styrofoam simply means any white foam of that type. This happens over and over in English: a brand name gets so common that it becomes the ordinary word.

Because it is a brand, you will see it written both ways: Styrofoam with a capital S in careful or legal writing, styrofoam with a small s in ordinary use. Both are normal. The technical name, if you ever need it, is expanded polystyrene, sometimes shortened to EPS.

It is a non-count noun. You say 'a piece of styrofoam' or 'a styrofoam cup', not 'two styrofoams'.

It also carries an environmental shadow now. It does not break down, it is awkward to recycle, and many places have banned it for food containers — so the word often turns up in news about waste and plastic bans.