where a periodic forces applied to a vibrating such that the frequency of the applied force becomes equal to the natural frequency of the vibrating body then there is an increase in the amplitude of the vibrating body then there is an increase in the amplitude of the vibrating body this phenomenon is called as resonance'.

Or

 When the frequency of the external driving force becomes equal to the natural frequency of oscillation of the body,then this body is thrown into natural vibration with large amplitude.
This phenomenon of producing large amplitude of oscillation by a small periodic driving force is called resonance'.