a force applied to a material (or structure) in any direction that is not perpendicular to the surface, (not normal to the surface).

The type of force that tends to make two adjacent parts of the same body slide relative to each other in a direction parallel to their plane of contact.

A strain, or change of shape, of an elastic body, consisting of an extension in one direction, an equal compression in a perpendicular direction, with an unchanged magnitude in the third direction.