In audio engineering, a fade is a gradual increase or decrease in the volume of a source, such as when a song is gradually reduced to silence at its end (fade-out), or gradually increases from silence at the beginning (fade-in). It can be used as either a verb or noun.
A fader is any device used to accomplish this task, especially when it is a knob or button that slides along a track or slot. A knob which rotates is usually not considered a fader, although it is electrically and functionally equivalent. A fader can be either analogue, directly controlling the resistance or impedance to the source; or digital, numerically controlling a digital signal processor (DSP).
A crossfader essentially functions like two faders connected side-by-side to each other, but in opposite directions. It allows a DJ to fade one source out while fading another source in at the same time. This is extremely useful when trying to beatmatch two phonograph records or compact discs.
In stage lighting, a fade is a gradual increase or decrease of the intensity of light projected onto the stage. The term fade-in refers to gradually changing the lighting level from complete darkness to a predetermined lighting level. A fade-out (also known as fade-to-black) refers to gradually decreasing the intensity of light until none is shining on the stage. A crossfade is when lighting levels are gradually altered from one setting to another.
In nearly all theatrical lighting designs, multiple lighting instruments are used to illuminate the stage at any one time. The instruments are controlled by a lighting technician from a dimmer board or lighting control panel. A fade refers to a change in illumination for the entire stage. Thus, the intensity of many lighting instruments are often altered with a single fade, especially with newer digital control systems like DMX, which uses MIDI to synchronise lighting and/or music.