This term is first proposed by Richard M. Stallman, founder of Free Software Foundation. This is not necessarily an ideological term, although it is indeed created to make a contrast to the notion of copyright. Its discourse, however, still works within the scope of the Copyright Law. In other words, the notion of copyleft can be summarized as: a way of sharing that works within the institution of the Copyright Law, albeit in ways different from the common practices. It is Stallman’s design that authors of free software programs can still claim the rights given to them by the Copyright Law and that the authors as the copyright holder can set the rules on how other people can use their works. Authors can, using the licensing terms, allow other people to perform modifications to the original work while at the same time require the authors of the resulting derivative work adopt the same licensing terms, so that it is assured that these works can continue their free circulation and use. This is the central idea of the notion copyleft: authors requiring their original and derivative works to be continuously available to the public. The classic example of such licensing terms is the GNU General Public License, or the GPL for short. For details on the terms, please refer to the GPL itself: http://www.gnu.org/licenses/licenses.html#WhatIsCopyleft