According to Hooke's law, within elastic limit, 

stress  strain

or            stress  = E Χ strain              or    Stress / Strain= E = a constant,

Where E is called as coefficient of elasticity or modulus of elasticity of a body which rely upon the nature of material of the body and the manner in which the body is deformed.  Therefore, particles of elasticity or coefficient of elasticity of a body is described as the ratio of the stress to the corresponding strain given, within the elastic limit.