Stress and Strain

These two quantities are often used interchangeably, but in physics they are totally distinct.

Stress is a measure of the force causing a material to deform. There are several ways you can stress a material. Almost always the units of stress are force per unit area:
So the unit of stress is the same as that for pressure.
Strain is a measure of how deformed the object becomes under the action of the stress. The strain is defined as a unit-less ratio:

If the original size is a length, the change will also be in length. The original size can also both be measured in terms of area or volume. This depends on the physical situation, as we will see in the following.

As long as strain is proportional to stress, we call the material elastic. Strangely, that designates diamond and steel as elastic materials, whereas rubber is quite inelastic. (Ever pull a rubber band really hard? It is quite different in length after the pull.)

Many students have trouble remembering that stress has dimensions of force per unit area, and strain is the dimensionless fractional change in size. For some, the following mnemonic device works: if you are under stress in your daily life, it is because of pressure, whereas if you strain to reach something, you change your length.