Knee (compressor)
The shape of a compressor's transfer curve around the threshold — a sharp corner or a gentle bend.
Plot input level against output and the line runs straight up to the threshold, then flattens according to the ratio. With a hard knee the change happens at a single point: one dB below threshold nothing happens, one dB above the full ratio kicks in. With a soft knee compression eases in over a range, perhaps 6–10 dB wide, with the ratio rising gradually. Hard knees sound controlled and audible; soft ones are more transparent but harder to place precisely.
dbx turned the soft knee into a trademark in the 1970s and called it OverEasy. In older designs the knee was not a setting but a side effect of the components — a variable-mu unit is soft by nature, a limiter hard by definition. In plugins it is now just one more knob.