It is the tendency to react slowly to an outside force, or to not return completely to its original state.
[1][2] For example, Bayliss and Robertson [3] and Hildebrandt [4] demonstrated that frictional stress in lung tissue is dependent upon the amount of lung expansion but not the rate of expansion, findings that are fundamentally incompatible with the notion of friction being caused by a viscous stress.
In many inert and living materials, the relationship between elastic and frictional stresses turns out to be very nearly invariant (something unaltered by a transformation).
[10][11][12][13] This close relationship between frictional and elastic stresses is called the structural damping law[1][2][4][14] or, sometimes, the constant phase model.
[5] The structural damping law implies that frictional losses are coupled tightly to elastic stresses rather than to viscous stresses, but the precise molecular mechanical origin of this phenomenon remains unknown.