Fader
Definition
A sliding control on a mixing console or DAW that adjusts the output level of a channel after all processing has been applied. The primary tool for setting balance between elements in a mix.
In Simple Terms
The volume slider for each track in your mix. After all your plugins have done their work, the fader sets the final volume for that instrument relative to everything else. The most important tool in mixing.
In Practice
After setting initial gain levels, the mix engineer uses faders to establish the relative balance of drums, bass, guitars, and vocals.