Strain is defined as:
An applied load or force
A deformation resulting from an applied load
An external force opposing an applied load
An internal force opposing an applied Load
Strain—Change in dimension per unit initial dimension. For tensile and compressive strain, a change in length is measured relative to the initial reference length.
Strain, or the change in length per unit length, is the relative deformation of an object subjected to a stress. Strain may be either elastic, plastic, elastic and plastic, or viscoelastic. Elastic strain is reversible. The object fully recovers its original shape when the force is removed. 
Phillips dental materials 12th edition page no 48, 51