-1) In audio technology, the term refers to the reduce in amplitude of an audio input waveform when another waveform is played at the same time. I don't really know how to explain it , but the best example would be heard in radios , when the song suddenly becomes softer when the DJ starts speaking. Thats side-chaining.
It is also used frequently in modern music too , especially trance and house music. An example would be Dash Berlin - Till the Sky Falls Down. Dance tracks (I don't know the right genre) uses them too , like Only Girl , where the pad is sidechained to the kickdrum.
-2 ) The most overused audio effect.
-3 ) The reason why modern music sounds so bad.
Sidechained Basslines are really wicked awesome!!
The ingredients to a generic trance tracks are..
1)Vengeance drum loops , hi hats , claps and kicks.
2)Nexus , and lots of presets from other VSTs.
3) Lots of LOTS OF SIDECHAIN!!!