It is commonly asserted that amorphous solids, such as glass have viscosity, arguing on the basis that all solids flow, to some possibly miniscule extent, in response to shear stress.

Advocates of such a view hold that the distinction bewteen solids and liquids is unlcear and that solids are simply liquids with a very high viscosity, typically greater than 1012 Pa·s.

This position is often adopted by supporters of the widely held urban myth that glass flow can be observed in old buildings.

However, others argue that solids are, in general, elastic for small stresses while fluids are not.

Even if solids flow at higher stresses, they are characterised by their low-stress behaviour.

Viscosity may be an appropriate characteristic for solids in a plastic regime.

The situation becomes somewhat confused as the term viscosity is sometimes used for solid materials, for example Maxwell materials, to describe the relationship between stress and the rate of change of strain, rather than rate of shear.