Stiffness refers to the resistance of an object to deformation when a force is applied. In materials science, it is a measure of how much a material will bend or stretch under stress. Stiff materials, like steel, do not easily change shape, while flexible materials, like rubber, can deform significantly. In engineering, stiffness is an important property that affects the performance and stability of structures. For example, the stiffness of a bridge determines how much it can sway in the wind or under the weight of vehicles. Understanding stiffness helps engineers design safer and more efficient structures.