Stress, in the context of physics and engineering, is a measure of the internal force that a material experiences per unit area when subjected to external forces or deformations. It is a fundamental concept in mechanics and material science.

There are several types of stress, including:

1. Normal Stress: Normal stress acts perpendicular to the surface of the material. It is often denoted by the symbol �σ (sigma). When the force is tensile (stretching), the normal stress is positive, and when the force is compressive (squeezing), the normal stress is negative.

2. Shear Stress: Shear stress arises when forces are applied parallel to the surface of the material, causing one layer of the material to slide over an adjacent layer. Shear stress is denoted by the symbol �τ (tau).

3. Von Mises Stress: Von Mises stress is a measure used in materials science to quantify the stress state at a point in a material. It combines both normal and shear stresses into a single equivalent stress value.

Stress is typically measured in units of force per unit area, such as Pascals (Pa) in the International System of Units (SI).