Quality Factor: The quality factor basically represents a figure of merit of a component (practical inductor or capacitor) or a complete circuit. It is a dimensionless number and defined as:

In series circuit it is defined as voltage magnification in the circuit at resonance

OR

It is also defined as the ratio of the reactive power of either the inductor or the capacitor to the average power of the resistor.

OR

In parallel circuit it is defined as equal to the current magnification in the circuit at resonance

OR

The quality factor or Q-factor of parallel circuit is defined as the ratio of the current circulating between two branches of the circuit to the current taken by the parallel circuit from the source.