Strain, in the context of stress, refers to the deformation or change in shape that occurs in a material when it is subjected to an external force. This force can cause the material to stretch, compress, or twist, leading to a measurable change in its dimensions. Strain is typically expressed as a ratio of the change in length to the original length of the material. There are different types of strain, including tensile strain, which occurs when a material is pulled apart, and compressive strain, which happens when it is pushed together. Understanding strain is essential in fields like engineering and materials science, as it helps predict how materials will behave under various loads.