native file format
A few of the standard image formats (TIFF and PNG, for example) provide support for an embedded alpha channel which represents up to 256 levels of transparency. Images with an embedded alpha channel can be ported to other applications while retaining transparency as long as the other application also supports alpha channels. Like a mask, the darkest areas of an alpha channel is most transparent, white areas are opaque, and shades of gray represent varying levels of transparency.
Also Known As: mask, selection
Elsewhere on the Web:
Anatomy of an Alpha Channel
The technical answer to "What is an alpha channel?" From Thomas Summerall, of Media Lab, Inc.