Alpha channel

A channel of information in some video clips and image files (in addition to the red, green, and blue color channels) that defines areas of transparency. An alpha channel can be represented as a grayscale image, with solid areas (100 percent opacity) shown in white, translucent areas shown in gray, and transparent areas (0 percent opacity) shown in black.

When you import a QuickTime video or an image file into a project, its alpha channel is recognized by Final Cut Pro. The alpha channel is then used to composite that image against other images in the viewer.