Group delay
How long a filter holds back each frequency — measured as how quickly the phase turns as frequency changes.
Formally it is the negative derivative of phase with respect to frequency, and the answer comes out as a time per frequency. If group delay is the same everywhere, the whole signal is delayed as a block and the waveform keeps its shape; that is linear phase. Analogue filters and digital IIR filters such as the biquad instead have a group delay that peaks around the cutoff, so some frequencies arrive a few milliseconds later than others.
It is rarely audible on a single track, but it is visible: a square wave through a high-pass filter comes out tilted and overshooting. In loudspeaker crossovers the woofer can end up lagging the tweeter, and there correction gets forced. Linear-phase EQ solves the problem by delaying everything equally, at the cost of long latency and reliance on delay compensation.