Define the relative permittivity of a medium.

Hint: When a charge is placed at any point, it creates a region around it in which another charge will experience coulomb force due to the charge. This region may extend up to infinity. The magnitude of force also depends on the medium in which the charge is placed. Coulomb force is maximum for vacuum and the force in another medium can be measured with respect to it.

Complete answer:
When two charges are placed in any medium at some distance, they experience force known as coulombic force. One of the factors affecting the magnitude of this coulombic force is the medium in which these are placed.
Permittivity is the property of a material or a medium that affects the coulomb force between two point charges when placed in that medium. Coulomb force is maximum for vacuum. Force in the other medium can be measured relative to it. The factor by which coulombic force between two charges is decreased relative to vacuum is defined as relative permittivity of the medium.
Relative permittivity can also be defined as the ratio of absolute permittivity of the medium to the permittivity of the vacuum. Relative permittivity is also known as dielectric constant.

Note:
Relative permittivity is a dimensionless number. Relative permittivity of the medium is also known as dielectric constant of the medium. The permittivity of vacuum is assumed to be unity.
When a dielectric is placed in an electric field, the electric field inside the dielectric volume is reduced measurably.