Viscous deformation refers to the deformation which varies linearly with time. The deformation is viscous up to a certain point.  Once the yield stress is exceeded, the deformation becomes plastic, where the deformation is not proportional to the stress.

Creep deformation is an example of viscoplasticity.

Viscoplasticity is studied in the analysis of crash tests in automobiles.  It is an important characteristic when choosing materials, which can withstand high loads.