Defined as a solid which is almost completely transparent for visible light glasses can be mineralic, polymeric and even metallic.
The factor which is essential for the optical transparency and which is common for all glasses is the amorphous molecular structure. The process responsible for light absorption is energy absorption to stimulate oscillations of the crystal grid of a solid. If a crystal grid is missing there is no absorption.
Amorphous structures can be created by cooling a melt very rapidly to a temperature below the melting point. In this case there is no time left for molecules to orientate into a crystal grid. The process is designated "shock freezing". Positions of molecules (or atoms) remain the same like in the liquid but loose the ability to move.
Usually only mineralic materials are just deignated as "glass". The dominating component is SiO2. Natural SiO2 minerals are generally crystallin. Shock freecing and, in most applications, additives which prevent from forming the crystal grid can create amorphous SiO2 in industrial processes.
If glass is produced by SiO2 without additives or contamination (>99% SiO2) the material is designated "quartz glas". If a fraction of about 15% Boroxide B2O3 is added it is called "Borsilicate glass". Both are almost completely inert against almost any etching substances including plasma. Therefore quartz glass or borsilicate glas are used for plasma chambers which are used for plasma etching.
Some polymeric materials such as PMMA or Polycarbonate can be amorphous and transparent. They are also designated "thermoplastic glasses".