Strain
Strain is the term applied to deformation (or change of shape) of an object when a load is placed on it. Delicate measuring instruments such as strain gauges may be needed to detect the amount of deformation.
The amount of deformation compared to the original shape is called strain and is represented by the letter 'e'.
This is a ratio which has no units:
Deformation is usually denoted by d or the symbol 
(delta) and generally refers to small deformations. For example, the strain in a beam under load is the change in length (L) compared to the original length (L) measured in millimetres.
