Definition: alpha channel

An 8-bit layer in a graphics file format that is used for expressing translucency. The additional eight bits per pixel serve as a mask and represent 256 translucency levels from entirely clear (0) to opaque (255), with levels in between representing the degree of haziness. See translucency, RGBA and HSLA.

Not All Formats Have an Alpha Channel
The TIFF, PNG and WebP graphics formats support the 8-bit alpha channel, whereas JPEGs have none. GIF supports a 1-bit channel, which means that an area marked as alpha can be either transparent or not but no levels in between.

Layered graphics formats such as Photoshop's native PSD format can have many alpha channels while the image is being composed. When converted to another graphics format, the alpha channels are reduced to one or none depending on the destination format. See alpha blending.

Alpha Channels Allow Overlapping

The alpha region (checkerboard) was created in order to copy the purple and green elements onto the center image. In this example, the alpha channel allows individual elements to overlap other elements, functioning similar to drawing and CAD programs (see vector graphics and graphics).