Technically speaking all solids can be regarded as amorphous if there are o sharp peaks in the wide angle X-ray scatterogram (Debye-Scherrer), i.e. long range order is absent. In metallic glasses or in glassy low molecular liquids it is usually a kinetically arrested metastable state which can be transformed in a thermodynamically stable state by annealing close to the melting point of the corresponding metal or liquid. In polymers lacking any long term symmetry (e.g. atactic liquid polymers) an equilibrium state with long term symmetry usually cannot be achieved. (Although proteins which may also lack long term symmetry in their chain can be crystallized in some cases but there the elementary unit is the whole molecule. Such an arrangement cannot be achieved in atactic vinyl polymers because of polydispersity). Amorphous solid polymers without symmetry are also in a metastable state which can be densified by annealing close to the glass transition temperature, but the long range order is still absent. Polymers with symmetry in the main chain (e.g. isotactic or syndiotactic vinyl polymers) are usually partially crystalline, where the amount of the crystalline phase depends on the cooling program but usually does not reach 100%.

Encyclopedia Britannica defines “Amorphous solid, any noncrystalline solid in which the atoms and molecules are not organized in a definite lattice pattern. Such solids include glass, plastic, and gel.

In addition to the terms amorphous solid and glass, other terms in use include noncrystalline solid and vitreous solid. Amorphous solid and noncrystalline solid are more general terms, while glass and vitreous solid have historically been reserved for an amorphous solid prepared by rapid cooling (quenching) of a melt.” https://www.britannica.com/science/amorphous-solid

Crystalline solids are solids that have a definite internal atomic structure that follows a regular, repeating pattern. Regular internal chemical structure also causes crystalline solids to have well defined melting points.

An amorphous solid does not have a definite geometric or crystalline shape. It is a solid in which there is no long-term order in the positions of the atoms. Most classes of solid materials can be found or prepared in an amorphous form. For instance, common window glass is an amorphous ceramic, many polymers are amorphous, and even foods such as cotton candy and cotton are amorphous solids.

An amorphous solid is any noncrystalline solid in which the atoms and molecules are not organized in a definite lattice pattern. Such solids include glass, plastic, and gel.

Non-crystalline solids can be divided in to two thermodynamically distinct classes: glasses and amorphous solids.

The definition of glass does not restrict either the composition or the optical properties of the material, implying a wide variety of different materials that are considered glass. In fact, theoretically, any crystalline solid that can be brought to a liquid state, can be forced into an amorphous solid state through rapid solidification via extraordinary cooling rates. This is easily observable by recognizing that quartz, a very common crystalline solid, has the same composition as silica glass (SiO2) but was cooled slowly enough to form long-range order. Non-silica glasses, in particular metallic glasses, can obtain unique electric, optical, or thermal properties from their crystalline counterparts through glassification. Non-metallic glasses can obtain similarly unique properties by adjusting elemental compositions and introducing dopants.  Glass is considered a vitreous supercooled liquid that is in a thermodynamically metastable state between the molten liquid state and the crystalline state. Different glass structures are influenced by the thermal history of the cooling process.

SOURCES : Wiki, Article The structure of glass: A phase equilibrium diagram approach

. https://eng.libretexts.org/Textbook_Maps/Chemical_Engineering/Supplemental_Modules_(Materials_Science)/Insulators/Glass_(Amorphous_Solids)

The description of glass as a supercooled liquid is technical inaccurate. It is an amorphous solid sharing similar properties as a random liquid - broad XRD peaks, for example.

Technically speaking all solids can be regarded as amorphous if there are o sharp peaks in the wide angle X-ray scatterogram (Debye-Scherrer), i.e. long range order is absent. In metallic glasses or in glassy low molecular liquids it is usually a kinetically arrested metastable state which can be transformed in a thermodynamically stable state by annealing close to the melting point of the corresponding metal or liquid. In polymers lacking any long term symmetry (e.g. atactic liquid polymers) an equilibrium state with long term symmetry usually cannot be achieved. (Although proteins which may also lack long term symmetry in their chain can be crystallized in some cases but there the elementary unit is the whole molecule. Such an arrangement cannot be achieved in atactic vinyl polymers because of polydispersity). Amorphous solid polymers without symmetry are also in a metastable state which can be densified by annealing close to the glass transition temperature, but the long range order is still absent. Polymers with symmetry in the main chain (e.g. isotactic or syndiotactic vinyl polymers) are usually partially crystalline, where the amount of the crystalline phase depends on the cooling program but usually does not reach 100%.

Amorphous word is quite old. It was used for formless solids even before the invention of X- ray diffraction. After the discovery of X- ray diffraction, it was found that solids which used to call amorphous were non crystalline. Since then,

amorphous and non crystalline are considered synonyms. All amorphous solids have absence of long range order and hence sharp diffraction peaks are not observed in their diffraction pattern.

Glass is a subset of amorphous solids. Glass is prepared from rapid cooling of the melt or by fast condensing vapours of constituent materials and has a glass transition temperature which is not observed in all amorphous solids. Being non crystalline in nature glass is also one kind of amorphous solids. It may be considered a special class of amorphous solid which has a glass transition temperature. Crystalline solids on the other hand are well known solids having periodic arrangements of atoms which show sharp diffraction peaks unlike amorphous solids which do not show such sharp peaks in their diffraction pattern.

Dr. A. Kumar

At the signalled paper we have tried to distinguish the often confused concepts of amorphous and glassy states:Article Aspects of the non-crystalline state