When the moving part of an oscillatory system is displaced from its equilibrium position and then set free, it oscillates to and fro about its equilibrium position with a frequency that depends on certain parameters of the system only. Such oscillations are known as free vibrations. The frequency with which the system oscillates is known as natural frequency.
When a body oscillates under the influence of an external periodic force, the oscillations are called forced oscillations. In forced oscillations, the body ultimately oscillates with the frequency of the external force.
The oscillatory system on which the oscillations are impressed is called driven and the system which applies the oscillating force is known as the driver.
The particular case of forced oscillations in which natural frequencies of the driver and the driven are equal is known as resonance. In resonant oscillations, the driver and the driven reinforce each other’s oscillations and hence their amplitudes are maximum.