In 1678, Robert Hooke obtained the stress-strain curve experimentally for a number of solid substances and established a law of elasticity known as Hooke’s law. According to this law, within elastic limit, stress is directly proportional to corresponding strain.
stress ∝ strain
stress/strain = constant (E)
The constant of proportionality E is a measure of elasticity of the substance and is called modulus of elasticity. As strain is a dimensionless quantity, the modulus of elasticity has the same dimensions (or units) as stress. Its value is independent of the stress and strain but depends on the nature of the material.