The top byte of a 32-bit pixel that is used for data other than colour. The alpha channel commonly holds mask data, enabling an image to be separated from its background for use in compositing.
An extra 8-bit, grayscale channel in an image - often used for creating masks to isolate part of an image. he degree of opacity of an image in computer graphics. Not the same as luminance. 
means of representing transparency in a computer graphic