Define the relative permittivity of a medium.

The amount by which the force between the charge (in presence of medium) is relatively less than that in a vacuum, is called the dielectric constant of that medium or relative permittivity (${ϵ}_r$)

${ϵ}_r=\frac{\text{Force between charges in air or vaccum}}{\text{Force between charges in medium}}$

$\therefore {\epsilon }_r=\frac{\epsilon }{{\epsilon }_0}$