What is the Q factor of an inductor ?

The Q factor of an inductor is a very important parameter.  The Q factor tell us how close the inductor to an ideal inductor. 

An ideal inductor is an inductor which has no losses.  That is, its series resistance is zero.  It is not possible to construct an ideal inductor as all inductors are made of wires which have resistances. 

Q factor is the ratio of the inductive reactance to the series resistance of the inductor  at a given frequency.

Q= XL/ R

The Q factor is an crucial parameter when designing resonant circuits as it will affect the damping.  Higher the Q factor, higher is the efficiency of the inductor.