High-pass filter
A filter that lets the treble through and attenuates everything below a cutoff frequency — the low-pass filter's mirror image.
The same two numbers describe it: the cutoff, where attenuation reaches 3 dB, and the slope, 6 dB per octave per filter order. On mixing desks and microphones the switch is usually labelled "low cut" and sits around 80 Hz, because most of what a vocal mic picks up below that is footsteps, floor rumble and air conditioning rather than music.
In mixing it is the most used tool nobody talks about: a high-pass on every track that isn't bass or kick drum clears the low end before anyone starts thinking about compression. Set to a few hertz, it removes DC offset. Digitally it is almost always built as a biquad, and like every minimum-phase filter it shifts phase around the cutoff.