Q factor
A measure of how narrow a filter is: the centre frequency divided by the bandwidth between the −3 dB points.
A bell filter at 1 kHz with Q = 1 touches roughly an octave around the centre; Q = 10 carves a narrow notch that barely disturbs the neighbours. The term comes from electrical engineering, where it describes how little energy a resonant circuit loses per cycle — high Q, long-ringing circuit. You can hear the same thing in audio: narrow filters ring longer in time.
On a synth filter Q usually goes by the name resonance and raises a peak right at the cutoff. Turn it far enough and the filter self-oscillates into a sine wave. The mixing rule of thumb "cut narrow, boost wide" exists because the ear hears a narrow peak as a fault and a narrow dip hardly at all.