Q Filter
Filter selectivity, i.e., the band of frequencies passed or rejected by the filter. The narrower the band of frequencies, the higher the Q, and the broader the band, the lower the Q. This is computed by Q = fc/f where fc is the center frequency and f is the band-width of the filter at the -3 dB points. [Bentley Nevada Corporation; Field of Rotating Machinery Measurement, Monitoring and Analysis]
A notch filter is a band-stop filter with a narrow stopband (high Q factor). [Wikipedia, Band-stop filter]
Due to the low resistance of their conductive walls, cavity resonators have very high Q factors; that is their bandwidth, the range of frequencies around the resonant frequency at which they will resonate, is very narrow. Thus they can act as narrow band-pass filter. Cavity resonators are widely used as the frequency determining element in microwave oscillators. Their resonant frequency can be tuned by moving one of the walls of the cavity in or out, changing its size. Wikipedia, Cavity Resonator
See Also