The damped natural oscillation of a system (e.g. spring-mass system or control circuit) is formed by oscillations or aperiodic movements that the system makes after external stimuli cease, i.e. when the system is left to itself. In the stable case the oscillations will decay.
Conversely, external excitation will cause forced oscillations. During this process the response function of the system has the same frequency as the exciting oscillation. However, the phase angle and amplitude are different.