Amorphous substances lack a rigid molecular shape, which means the atoms and molecules that make up the substance are not "locked" into place but can move about. This is in contrast to crystalline substances in which the atoms are firmly "locked" into place.
All gases and liquids are amorphous. They have a very low density compared to solids and are generally held together by weak bonds, such as hydrogen bonds, dipole-dipole bonds and Van der Waals forces. In addition, gases and liquids are fluid, which means that they assume the shape of the container they are placed in.
An amorphous solid is unlike a crystalline solid in that the pattern of its atoms or molecules does not periodically repeat in three dimensions. An amorphous solid is distinguished from a liquid by its viscosity, the measure of a fluid's resistance when put under stress. If a substance has a shear viscosity higher than 10^14.6 poise, it is considered an amorphous solid rather than a liquid. Thus, honey which has a viscosity of 200,000 poise is considered a thick amorphous liquid, whereas glass, which has a viscosity of up to 10^22 poise at room temperature, is considered an amorphous and viscous solid.
There are a number of different amorphous solids which one might come across, in both everyday usage and in various technological fields. By far the most common amorphous substances are the varieties of glass and glassy polymers (plastics). Other uses for amorphous solids include the dielectric solids and protective coatings used in integrated circuits. The process of Xerox photocopying also depends upon an amorphous semiconductor.