State-variable filter (SVF)
A filter built from two integrators in a feedback loop, delivering lowpass, bandpass and highpass outputs at the same time from one circuit.
The topology comes from analogue computing, where differential equations were solved by wiring integrators into loops. Each node in the loop carries its own version of the signal: bandpass after the first integrator, lowpass after the second, highpass at the input. Add high and low together and you get a notch thrown in for free. Cutoff and resonance are also independent of each other, which other topologies cannot always claim.
Oberheim's SEM of 1974 made it a synth sound in its own right: 12 dB per octave, gentler than Moog's ladder, with a knob that swept between filter types. Hal Chamberlin published a digital version in 1980 that became standard in early DSP synths, though it went unstable near the top of the spectrum. Modern designs using zero-delay feedback have fixed that.