Quality Factor

Quality factor or Q , is a measure of the quality of a resonance circuit. The higher the Q value, the narrower the bandwidth of the resonance.
A higher Q indicates also a lower rate of energy dissipation relative to the oscillation frequency

Q = \omega \times \frac{\mbox{Energy Stored}}{\mbox{Power Loss}}

Q for parallel RLC circuit:

Q = \frac {R} {\sqrt\frac{L}{C}}

Q for serial RLC circuit:

Q = \frac{1}{R} \sqrt{\frac{L}{C}}

For complex impedance:

Q = \left  \frac{X}{R} \right