Hook’s law:
The strain produced in a body by the stress applied to it is directly proportional to the stress within the elastic limit of the body. 
           
Thus, $\text{stress} \propto \text{strain}$
Or, $\text{stress} = \text{constant} \times  \text{strain}$
Or, $\frac{(\text{stress} )}{\text{strain}}  =   \text{constant}$

Elastic Limit:

The greatest stress that can be applied to a material without causing permanent deformation is called the elastic limit.

The stress point at which a material, if subjected to higher stress, will no longer return to its original shape. Brittle materials tend to break at or shortly past their elastic limit, while ductile materials deform at stress levels beyond their elastic limit.