An amorphous solid is a solid in which there is no long-range order of the positions of the atoms. (Solids in which there is long_range atomic order are called crystalline solids.) Most classes of solid materials can be found or prepared in an amorphous form. For instance, common window glass is a ceramic, most polymers are amorphous, and even some amorphous metallic alloys can be prepared under special processing conditions (such as rapid solidification and ion implantation). Glass is often referred to as a 'super-cooled' liquid.

There are many ways in which amorphous solids can be made. If a liquid is cooled rapidly enough to avoid crystallization, an amorphous solid called a glass is formed below the glass transition temperature. Amorphous solids produced by other routes, such as ion implantation are, technically speaking, not glasses. In common parlance, the term glass refers to amorphous oxides, and especially silicates (compounds based on silicon and oxygen). To avoid confusion, other types of glass often are referred to with a modifier, such as the term 'metallic glass' to refer to amorphous metallic alloys.

It is difficult to make a distinction between truly amorphous solids and crystalline solids in which the size of the crystals is very small (less than two nanometers). Even amorphous materials have some short-range order among the atomic positions (over length scales of about one nanometer). Furthermore, in very small crystals a large fraction of the atoms are located at or near the surface of the crystal; relaxation of the surface and interfacial effects distort the atomic positions, decreasing the structural order. Even the most advanced structural characterization techniques, such as x-ray diffraction and transmission electron microscopy, have difficulty in distinguishing between amorphous and crystalline structures on these length scales.

The transition from the liquid state to the glass, at a temperature below the equilibrium melting point of the material, is called the glass transition. From a practical point of view, the glass transition temperature is defined empirically as the temperature at which the viscosity of the liquid exceeds a certain value (commonly 10¹³ Pascal_seconds). The transition temperature depends on cooling rate, with the glass transition occurring at higher temperatures for faster cooling rates. The precise nature of the glass transition is the subject of ongoing research. While it is clear that the glass transition is not a first_order thermodynamic transition (such as melting), there is debate as to whether it is a higher_order transition, or merely a kinetic effect.

Some examples of amorphous solids are glass and plastic.