Shelving filter
A filter that boosts or cuts everything above or below a corner frequency by a fixed amount, so the curve levels off like a shelf.
A low-pass filter keeps attenuating the further you go; a shelf stops at a set level. +4 dB above 8 kHz means +4 dB at 10 kHz and at 18 kHz too. The bass and treble knobs on a stereo amplifier are almost always shelving filters, usually in the circuit Peter Baxandall published in 1952.
In a digital EQ it is typically a biquad worked out from Robert Bristow-Johnson's Audio EQ Cookbook, with steepness set as Q or "shelf slope". Push Q high and a small bump appears just before the shelf edge — a bug to some, and precisely the sound that sells an analogue emulation to others.