What is meant by resonance?

Hint: When a vibrating body is subjected to an external vibration and if the natural frequency of vibration of the body matches with the frequency of applied vibration, then the body starts to vibrate at large amplitude.

Complete step by step solution:
All things in our universe are in a state of constant motion with respect to one another. All objects vibrate with a certain frequency called the natural frequency which is characteristic of that particular body.
A phenomenon is observed when a body is subjected to external vibrations. If the frequency of the applied vibration is equal to the natural frequency of that body then the body starts vibrating with a very large amplitude. This phenomenon is known as resonance. The frequency at which resonance takes place is called resonant frequency. We can say that the resonant frequency of a body is basically equal to the natural frequency of the body.

There are different types of resonance:
1. Acoustic resonance: It occurs in an acoustic system like a musical instrument where the sound waves having frequency equal to the natural frequency of the system get amplified.
2. Mechanical resonance: When frequency of oscillation of a mechanical system matches its natural frequency then it starts to vibrate at large amplitudes. It might be considered an undesirable effect here as moving parts can get damaged by vibrating at large amplitudes.
 3. Electrical resonance: This resonance occurs in an electric circuit involving capacitors and inductors. At a particular frequency of alternating current, the capacitive and inductive reactances match each other.

Note: Resonance can occur in a building during an earthquake which can cause huge damages. Soldiers must break their steps while crossing a bridge as resonance can make the bridge vibrate to the extent of large damages.