An extra layer of information stored in a digital picture to describe transparency or opacity. For each pixel, the alpha channel stores an extra value called alpha, in addition to its red, blue and green values, which indicates the degree of transparency of that pixel.
The display software then mixes the colour of this pixel with the background colour in proportion to its alpha value (so an alpha value of 0.5 would display half foreground and half background), a process called alpha blending.
An alpha channel enables special effects such as blurring or tinting of the background as a transparent object passes across it, and fog or mist effects to suggest distance. Alpha blending is supported as a hardware function by advanced graphics accelerators (AGP).