The quality factor or Q factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is. It is a measure of the performance of a coil, capacitor, or inductor in terms of its losses and resonator bandwidth. The Q factor is defined as the ratio of the initial energy stored in the resonator to the energy lost. The higher the Q factor, the less energy is lost, and the more stable and efficient the system becomes.

The Q factor is used with many RF tuned circuits or elements to indicate their performance in an oscillator or other form of resonant circuit. The Q factor of oscillators varies substantially from system to system, depending on their construction. Systems for which damping is important, such as dampers keeping a door from slamming shut, have Q near 1/2. Clocks, lasers, and other resonating systems that need either strong resonance or high frequency stability have high quality factors. Tuning forks have quality factors around 1000.

A system with low quality factor (Q < 1/2) is said to be overdamped. Such a system doesnt oscillate at all, but when displaced from its equilibrium steady-state output, it returns to it by exponential decay, approaching the steady-state value asymptotically. As the quality factor decreases, the slower decay mode becomes stronger relative to the faster mode and dominates the systems response resulting in a slower system.

The Q factor is also important in filters and amplifiers. In a filter, the Q factor determines the sharpness of the cutoff between the passband and the stopband of the filter. A high-Q factor filter has a very sharp cutoff, while a low-Q factor filter has a more gradual cutoff. In an amplifier, the Q factor influences the selectivity and gain of the amplifier.

In the field of acoustics, the Q factor is a measure of the sharpness of resonance in musical instruments. A high-Q instrument, such as a violin, can produce a pure and sustained tone, while a low-Q instrument, such as a maraca, produces a broader and less sustained sound. The Q factor can also affect the sound quality and resonance of speakers and microphones.