The letter Q, often called the quality factor, is used in discussions of clocks and oscillators. The concept of Q is applicable to any system which exhibits energy dissipation and periodic behavior. It is a measure of energy loss; actually, a self-timed measure of the rate of energy loss. Thus springs have Q ('vibrators'); pendulum clocks have Q (oscillators); atomic clocks have Q (resonators); planets have Q ('rotators').
For a general description of Q see the Wikipedia page: en.wikipedia.org/wiki/Q_factor
Here is an image showing where, in the context of electronic resonance, Q was first used:
Letters in the Quarterly Journal of the Royal Astronomical Society, by Bertha Jeffrys, David McMullan, Peter Fellgett, Ken Smith:
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