The Force transmitted across any section, divided by the area of that section, is called intensity of stress or stress. Forces acting on a material cause a change in dimensions and the material is said to be in a state of stress.
Stresses which are normal to the plane on which they act are called direct stresses and either tensile or compressive
The symbol σ is called SIGMA
Strain is a measure of the measure of the deformation produced in the member by the load. The fractional change in a dimension of a material produced by a force is called the strain. For a tensile or compressive force, the strain is the ratio of the change of length to the original length. The symbol used for the strain is ε (Greek epsilon). Tensile strain will be positive While compressive strain will be negative.
NOTE: Strain is dimensionless and is often expressed as a percentage