Three models can describe how a solid responds to an applied stress:

Elasticity - When an applied stress is removed, the material returns to its undeformed state.

In materials science, the strength of a material is its ability to withstand an applied stress without failure.

Plastic deformation is retained after the release of the applied stress.

Below a critical value of applied stress, a material may exhibit linear viscoelasticity.

Increasing the applied stress to move the dislocation increases the yield strength of the material.

And by fluid we mean something very specific - it's any substance that flows under an applied stress.

The grain size is known to decrease as the applied stress is increased.

The higher the applied stress to move the dislocation, the higher the yield strength.

When the back stress is the same magnitude as the applied stress, the material no longer creeps.