Hooke's law:

Hooke's law states that, within the elastic limit stress developed in body is directly proportional to strain produced in it. It can be defined as the stress imposed on a solid is directly proportional to strain produced, within elastic limit of it. The maximum stress up to which a body exhibits the property of elasticity is called as elastic limit or limit of elasticity. Modulus of elasticity, known as elastic modulus or Young's Modulus, is a measure of how a material or structure will deform and strain when placed under stress. It can be denoted by Y. The modulus of elasticity, or Young's modulus, is the ratio of change in stress to change in strain during elastic deformation. The modulus of elasticity describes the relationship among the stress applied to a material and its corresponding strain. 

Stress can be defined as a force applied over a unit area, with typical units of pounds per square inch (psi) or Newton per square meter also known as pascals (Pa). Strain is a measure of the amount which a material deforms when stress is applied and is computed by measuring the amount of deformation when under stress, as compared to the matter's original dimensions. Modulus of elasticity is based on Hooke's Law of elasticity and can be computed by dividing the stress by the strain. Elasticity is the ability of a material to return to the original state of it or dimensions after a load or stress, is removed. Elastic strain is reversible, meaning the strain will disappear after the removal of stress and the material will return to its original state. Materials which are exposed to intense levels of stress can deform to the point where the stress and strain do not behave proportionally, and the material will not return to its original dimensions. This is referred to as  plastic strain or plastic deformation.