Hooke's law of elasticity is a principle in physics that states that the force required to extend or compress a spring by some distance is proportional to that distance. It is named after the English physicist Robert Hooke, who first stated the law in 1678. Hooke's law is often written as F = -kx, where F is the force applied to the spring, x is the distance it is stretched or compressed, and k is the spring constant, a measure of the stiffness of the spring.