In damped oscillation, suppose, an oscillator having mass M and force constant k is acted upon by a resistive force F0 .
It is found that the resistive force is proportional to the velocity (v) of the oscillator. That is Fb=bv, where b is a constant and is called the damping factor. If x be the instantaneous displacement of the oscillator, then Mdt2d2x=−kx−bv
So, a particle oscillating under a force F=−kx−bv is a damped oscillator.