Copyleft refers to a concept invented by Richard Stallman of the Free Software Foundation around 1984 or 1985: the application of copyright laws to ensure the perpetual availability to the public of a certain piece of information and all its derivative works.
Initially only designed for software distribution, the concept is now also being used for other types of material.
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. This differs from some other Open Source licences which allow proprietary works to be made from the software.
Copyleft is sometimes referred to as a 'Public Domain Virus', in that any works based on a copylefted work must themselves be copylefted. Advocates of copyleft point out that with most proprietary licenses creating derivative works is simply not allowed.
The term "Copyleft" comes from the phrase "Copyleft--all rights reversed", which Don Hopkins wrote in a message to Stallman in 1984 or 1985 and which is intended as a double pun on the phrase "Copyright--all rights reserved".
Free software licenses which are examples of copyleft licenses include the GNU General Public License and the BSD License. Copyleft licenses for publications include the Open Content License, and the GNU Free Documentation License. The latter is being used for the contents of this encyclopedia.