alpha channel

An 8-bit layer in a graphics file format that is used for expressing translucency (transparency). The extra eight bits per pixel serves as a mask and represents 256 levels of translucency. For example, TIFF and PNG are graphics formats that support the 8-bit alpha channel. The GIF89a format supports a 1-bit alpha channel that allows one color on the palette to be translucent.

Graphics formats that are native to their image editing application may support multiple alpha channels. For example, Photoshop's native PSD format can have a total of 24 channels, including color channels (RGB, CMYK, etc.) and alpha channels. When exported to another graphics format, the alpha channels are reduced to one or none depending on the format. See alpha blending.