Definition: When the frequency of the external periodic force applied to a body is exactly equal to (matches) natural frequency of body, the body vibrates with maximum amplitude, the effect is known as resonance.
Examples: 1) Bridge may collapse in earth quake if forced frequency of earth quake becomes equal to the natural frequency of the bridge.
2) Use of musical instruments like flute, harmonium, sitar, violin, guitar.
3) Radio receiver set.