Mechanical stress refers to the internal forces that develop within a material when it is subjected to external loads or forces. It is measured as the force applied per unit area and can lead to deformation or failure of the material. Common types of mechanical stress include tensile stress, compressive stress, and shear stress, each affecting materials differently. When materials experience mechanical stress, they may undergo changes in shape or size, known as strain. Understanding mechanical stress is crucial in fields like engineering and materials science, as it helps in designing structures and components that can withstand various loads without breaking or deforming.