It is a device capable of storing a huge amount of electrical charges on its plates. Thus it is a device for storing energy in the electric field between its plates.
Generally it consists of two insulated conductors close together, and carrying equal and opposite charges. The conductors are called the plates of the capacitor.
It is defined as the ratio of the magnitude of the charge q on either plate to the potential difference V between the plates
C = q/V
Capacitance C of a capacitor is a measure of its capacity of storing charges.
Capacitance of a capacitor depends upon its shape, size and the relative permittivity of the medium between the plates.
C = q/V
If q = 1 coulomb and V = 1 volt, then the capacitance C = 1 coulomb/volt.
It is also called a farad (F) which is defined as that a charge in coulombs which would develop a potential difference of 1 volt across the two conductors of the condenser
1 farad = 1 coulomb/1 volt