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
The Q factor is scaled so that a factor of 1/2 or less dissipates too much energy to carry out even a single oscillation, and the higher the number (1, 100, 1,000,000, etc.), the more oscillations result without addition of energy.
The Q factor arises in astrophysics, for example, regarding orbital resonances.
A Q factor is used as a measure of some of the effects of tides on orbits and body rotations, called the tidal q factor (tidal Q or tidal quality factor). This is a Q-factor-style measure of the ratio between energy dissipated by an oscillation and a measure of the deformation during an oscillation. Like the above, high tidal Q factors (a thousand, a million) result in very slow damping/dissipation, and lower numbers leading to quicker dissipation. Such a tidal Q factor depends upon the specific oscillation, e.g., a body's tidal deformation due to its rotation under the influence of a nearby mass (e.g., a planet and its host star) has a corresponding tidal Q factor, and if it also undergoes a deformation due to an eccentric orbit, that oscillation has a not-necessarily-the-same tidal Q factor.