What's "quality factor" in oscillation ?

[abcdefg10645]

I want to know why and how it was defined ?

Is it useful in some ways ?

[f95toli]

Quality factor ("Q") is only useful for damped oscillations; it is basically measure of how long it takes for the oscillation to die out or -equivalently- how narrow the oscillation is in frequency space (e.g a very "selective" filter has a high Q value).

have a look at this page
http://www.fonema.se/qval/qval.htm

And yes, it is used a lot.