Zero-delay feedback (ZDF)
A digital filter technique that resolves feedback within the same sample, so resonance and modulation behave as they would in an analogue circuit.
In an analogue filter, feedback acts instantly. The straightforward digital translation cannot use a value it has not yet computed, so it slips a one-sample delay into the feedback path. That sounds harmless, but the resonance lands in the wrong place, the filter turns unstable near high cutoffs and fast sweeps sound stiff. ZDF instead solves the equation for the current sample directly, using trapezoidal integration and a structure-preserving rewrite of the circuit. Nonlinear filters usually need iteration, Newton–Raphson for instance.
Vadim Zavalishin of Native Instruments collected the method in his freely available book The Art of VA Filter Design in 2012. Around then "ZDF" became a selling point on plugin boxes, and u-he's Diva (2011) earned a reputation for sounding like hardware and eating CPU to match.