The tidal Q (for tidal Q factor or tidal quality factor) is a number associated with an orbiting body characterizing the rate at which tidal force converts kinetic energy to heat. It is the maximum potential energy that occurs during tidal distortion divided by the energy lost over the course of a cycle (the tidal period), typically this ratio multiplied by 2π. For the tidal Q associated with an orbit, the cycle lasts for an orbital period (adjusted if necessary to accommodate the body's rotation). The "lost" energy may be converted to heat, a source of heat page in the body, such as a moon. A tidal Q much greater than one indicates the conversion is occurring very slowly. A tidally locked moon in a perfectly circular orbit theoretically has an infinite tidal Q (no energy converted) since there is no change to the stress and strain in the course of the orbit. An eccentric orbit has a varying tidal force, more when the moon is nearer, and internal friction from the stress generates heat, which can be significant, i.e., keep a moon measurably warmer than would otherwise be expected. Also, in such an eccentric orbit, the locking is imperfect since the moon rotates rather uniformly once per period, but the direction facing the host planet does not rotate uniformly. Viscosity may contribute to the tidal Q of gas planets, but inertial waves can play larger role.
Note that a body may be affected by than one tide mechanism (e.g., the Earth's tides due both to the Sun and the Moon), each with its Q. Also, tides affect particular portions of a body, such as the Earth's sea tides, atmospheric tides, and tides within Earth's solids. Q may be subscripted to indicate it is the quality factor specific to some mechanism and/or portion of the body.
The terms Q factor (quality factor or just Q) are used more generally for oscillators and their damping and this use can be thought of as treating the tidal cycle as an oscillation.