CopyLeft
From What's new, new media?
The term CopyLeft describes the tight relation of a piece of copyrighted work and a certain type of license. The license permits you to modify the work. If you redistribute that modified version you are required to do so under the original license. This license gives you the freedom to modify the work and redistribute it, if you do so under the same license. A common example is the GNU Free Documentation License[1].
