Low-pass filter
A filter that lets the lows through and attenuates everything above a cutoff frequency.
Two numbers describe one: the cutoff, where attenuation reaches 3 dB, and the slope, how steeply it falls away afterwards. Each pole adds 6 dB per octave, so a four-pole filter cuts at 24 dB per octave. Resonance boosts the region right at the cutoff, making the filter whistle — or self-oscillate if you push it far enough.
In subtractive synthesis the low-pass filter is the star. Robert Moog's transistor ladder, patented in 1969, produced the round bass that every soft synth still imitates. The same kind of filter sits quietly in front of every A/D converter, removing anything above the Nyquist frequency before it can fold back down as aliasing. In software it is usually built as a biquad.