Notch filter
A filter that removes a very narrow band of frequencies and leaves everything else nearly untouched.
It is the mirror image of a band-pass filter: instead of letting a thin slice through, it cuts that slice out. The higher the Q, the narrower the hole. In analogue electronics the twin-T network of three resistors and three capacitors was the classic design; in DSP a biquad with the right coefficients does the job.
Two uses dominate. Mains hum at 50 or 60 Hz, often caused by a ground loop, gets removed by a series of notches on the fundamental and its harmonics. And on stage, feedback suppressors hunt down whichever frequency has started to howl and drop a notch right on it. Filtering out the symptom is, of course, always easier than fixing the wiring.