Quack, quack, what exactly is the definition?

It's when you reduce the background sound while you are talking over it.

Some DAWs or filters provide for "auto-ducking".

If, for example, you have a lengthy voiceover over music, you might want the music a little hotter in gaps between the voiceover, a little quieter during VO.

Often done "live" and "automatically" with a DJ talking over background music.

When overdone, the depth of ducking, and the speed of response means you get a wall of sound from the radio at constant (high) volume - especially on "youth" music radio stations.

Easily done feeding a compressor from a side-chain, keeping the quacking above the music.
Quack, Quack, Quack, Quack, Quack, Quack, Quack, Quack, Quack, Quack

Although aften incorporated into audio studio setups, you can buy 'audio ducking' devices separately --see here for example: Rolls Corporation - Real Sound - Products DU30b Mic-Preamp/Audio Ducker (http://www.rolls.com/product.php?pid=DU30b)

Click on the 'manual' and there's a great deal more detail, including the full circuit diagram for the device.

Never been quite sure why some manufacturers are prepared to 'bare all,' as it were, with their information, and with others it's like trying to get blood out of a stone!