VCF (voltage-controlled filter)
The analog synth's filter, with a cutoff set by a control voltage rather than just a knob, so the timbre can move while the note plays.
The VCF sits between the VCO and the VCA in the classic chain. Voltage control means an envelope, an LFO or the keyboard can push the cutoff around: an ADSR that opens the filter fast and closes it slowly produces the "wow" at the start of every synth bass. Keyboard tracking lets the cutoff follow pitch, so high notes don't go dull.
The filter design is a large part of what a synth sounds like. Moog's transistor ladder is a four-pole low-pass filter at 24 dB per octave; Oberheim's SEM used a 12 dB state-variable filter that could switch between low-pass, band-pass and high-pass. The difference is audible, and synth forums have spent decades trying to describe it with words like "creamy" and "screamy".