The term amorphous is derived from the Greek word amorphos, which means "without form". The condition for the amorphous state is that the atoms cannot arrange themselves regularly (i.e. the viscosity must exceed a certain value) and that no crystallization takes place.

Amorphous materials are those in which the atoms do not form an ordered structure but an irregular pattern. They only have a near order, but no far order. Distant order, i.e. a regular arrangement of atoms beyond their neighboring atoms, is characteristic of crystals. Crystals are regularly structured materials.

Like liquids, amorphous materials are isotropic, i.e. they have no preferred direction. The reason for this is the lack of long-range order. Amorphous material is often unstable and can crystallize under certain conditions.

The amorphous state is often produced by rapid cooling of a material so that the atoms or molecules do not have time to arrange themselves in an ordered structure.

However, there are also other ways to create the amorphous state. The crystalline order can also be destroyed by mechanical deformation, ion bombardment or radiation.

Not all materials can be produced in amorphous form; some materials require very rapid cooling to reach the amorphous state.

When deciding between a crystalline and an amorphous substance, it is important to know that the amorphous state almost always has a lower density than the same material in its crystalline form.

This is because the atoms in an amorphous substance have a lower packing density, i.e. they are not as densely packed as in a crystalline form. The amorphous state is therefore considered metastable, which means that it is less stable than a crystal of the same material.

When an amorphous substance is heated, spontaneous crystallization can occur, transforming it into a more stable state. This is called "crystallization" or "recrystallization". If this does not happen first, there is also a direct transition from an amorphous substance to a liquid without a classical phase transition.

Glass is a typical amorphous material. Quartz glass is the amorphous form of silicon dioxide (SiO2). One of its crystalline forms is called quartz.

Amorphous metals are produced industrially in the form of thin layers using rapid solidification technology. The most important applications are magnetic materials, soft magnetic alloys (Fe, Ni, Co) and amorphous solder foils.

Amorphous silicon is a non-crystalline form of the pure semiconductor silicon and is mainly used for thin-film solar cells.

Amorphous carbon is obtained by chemical vapor deposition. Obsidian is a natural amorphous material of volcanic origin.

Amorphous thermoplastics (plastics) are, for example, polystyrene (PS), polyvinyl chloride (PVC) or polycarbonate (PC).