Stress in a solid body is defined as the internal forces developed due to externally applied forces, per unit area. It is denoted by the symbol σ and is typically measured in units of force per unit area, such as pascals (Pa) or newtons per square meter (N/m^2).

Stress is a fundamental concept in mechanics and is used to analyze the behavior and deformation of solid materials under external loads. It describes the internal distribution of forces within a material and is an important parameter in determining its mechanical properties.

There are different types of stress that can occur in a solid body, including normal stress and shear stress. Normal stress, also known as axial stress, is the force per unit area acting perpendicular to the cross-sectional area of the material. Shear stress, on the other hand, is the force per unit area acting parallel to the cross-sectional area.

The stress in a solid body is a result of the external forces applied to it. These external forces can be in the form of loads, such as compression, tension, bending, or torsion. When an external force is applied to a solid body, it generates internal forces within the material that resist the deformation caused by the applied load. These internal forces create stress in the material.

The stress distribution within a solid body can vary depending on the geometry, material properties, and boundary conditions. It can be uniform, where the stress is the same at every point within the material, or it can be non-uniform, where the stress varies across the material.

In conclusion, stress in a solid body is defined as the internal forces developed due to externally applied forces per unit area. It is an important parameter in analyzing the behavior and deformation of materials under external loads.