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Copyleft refers to the application of copyright law to ensure public freedom to manipulate, improve, and redistribute a work of authorship and all derivative works. The concept was invented by Richard Stallman in 1984 for the distribution of computer software, althought it is now also being used for other types of material. The term "copyleft" came from the phrase "Copyleft--all rights reversed", written by Don Hopkins in a message to Stallman in 1984. It is a double pun on the phrase "Copyright--all rights reserved".
In copyleft, the copyright holder grants an irrevocable license to the recipient of a copy, permitting the redistribution (including sale) of possibly modified further copies, under the condition that all those copies carry the same license and are made available in a form which facilitates modification.
Free software licenses that use copyleft include the GNU General Public License, the GNU Lesser General Public License, the Mozilla Public License, and the Q Public License. Other free software licenses, such as those used by the BSD operating systems and the X Window System, are not copyleft licenses because they do not extend to derivative works and include no requirement to make source code available. A longstanding issue of debate is which class of license provides a larger degree of freedom. It is often argued that the copyleft licenses attempt to maximize the freedom of the initial recipient, whereas the latter licenses try to maximize the freedom of all potential recipients in the future.
Copyleft licenses for materials other than software include the Open Content License, and the GNU Free Documentation License. The latter is being used for the content of Misplaced Pages. The Free Art license is a license that can be applied to any work of art.
Copyleft licenses are sometimes called viral copyright licenses because any works derived from a copylefted work must themselves be copylefted. This term is considered derogatory, as it compares copylefted works to harmful computer viruses. However, the analogy between copyleft and computer viruses is not close; as advocates of copyleft point out, computer viruses infect computers without the awareness of the user, whereas the copyleft actually grants the user certain permissions to distribute modified programs, which is otherwise not allowed under copyright law or by most proprietary licenses. Furthermore, copyright itself would be "viral" in this sense, since any works derived from a copyrighted work must have permission from and obey any conditions set by the original copyright holder.
The concept of copyleft arose when Stallman was working on a Lisp interpreter. Symbolics asked to use the Lisp interpreter, and Stallman agreed to supply them with a public domain version of his work. Symbolics extended and improved the Lisp interpreter, but when Stallman wanted access to the improvements that Symbolics had made to his interpreter, Symbolics refused. Stallman then proceeded to create a software license that would prevent this behavior.
Copyleft-like ideas are increasingly being suggested for patents, such as open patent pools that allow royalty-free use of patents contributed to the pool under certain conditions (such as surrendering the right to apply for new patents that are not contributed to the pool).