The true definition of ducking is when you attenuate one audio signal to give way for another.
The side chain is the curcuit that controls the compressors attack, deacy, ratio etc...
One classic example of ducking is on the radio when the presenter talks over the song playing and the volume of that song is automatically 'ducked' to allow the voice come through. This is normaly done using the sidechain (it can be done manualy by riding the fader but ducking with a compressor is easier).
You can also use the side for de-essing and other similar frequency removal/enhancement tricks and techniques.
Archaic is describing normal use of compression and is not really true ducking.
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