The Q factor (quality factor) is a quality of an oscillator reflecting how damped it is, the higher the Q factor, the less damping and the more the oscillator will resonate. For example, a pendulum's friction lowers its Q factor. The Q factor is defined as:
Energy Stored Q = 2π x ———————————————————————————— Energy dissipated per cycle
In astrophysics, it is a factor in orbital resonances, for example.