Stands for auxiliary send. An additional output of an audio mixer. This can be thought of as a mixer within a mixer. Each channel has a rotary knob (usually) that will allow the signal of that channel to be sent to the auxiliary send output of the mixer (just as raising the fader allows the signal of that channel to be sent to mixers main outputs). Aux sends are typically used to feed effects processors or monitor speakers, but have countless uses.
The signal for each channel can be derived either pre fader or post fader. In the case of a prefade aux send, the signal from the channel to the aux send is not effected by the movement of the corresponding channel's fader. These are often used when mixing monitors from FOH. In a post fade scenario, the signal to the aux send is adjusted proportionately as the fader is moved. This is useful when fading out music or effects, or when using an aux send to feed sub woofers or front fills.
The signal for each channel can be derived either pre fader or post fader. In the case of a prefade aux send, the signal from the channel to the aux send is not effected by the movement of the corresponding channel's fader. These are often used when mixing monitors from FOH. In a post fade scenario, the signal to the aux send is adjusted proportionately as the fader is moved. This is useful when fading out music or effects, or when using an aux send to feed sub woofers or front fills.