Glass, a hard, brittle, and usually transparent material. As a scientific term, the word glass is often used to refer to any rigid substance that is amorphous (not made up of crystals), with the exception of plastics and certain other substances containing carbon. Glass is an extremely useful material. It is made into a large variety of familiar products, including windows, jars, mirrors, light bulbs, lenses, and tableware.

Glass is often classified according to its composition. Glass is also classified according to form, use, properties, or method of manufacture. For a short description of different kinds of glass, see, in this article, the section titled Some Kinds of Glass and Glass Products. Some kinds of glass, such as obsidian, occur naturally.