Copyleft

''This page describes the legal concept of Copyleft. OSGeo does not restrict projects to use the GNU GPL or any other license that implements a Copyleft effect. For more information on OSGeo's license policy please read the FAQ and Article 1 §2 of the bylaws.''

Legal Definition
Copyleft is a legal measure to protect software from becoming proprietary. Copyleft leverages the considerable legal powers that have been implemented in the concept of Copyright. Thus Copyleft does not oppose Copyright or turn it around but instead Copyleft is enforced by the full power of Copyright.

To copyleft a program, we first state that it is copyrighted; then we add distribution terms, which are a legal instrument that gives everyone the rights to use, modify, and redistribute the program's code or any program derived from it but only if the distribution terms are unchanged.

Thus, the code and the freedoms become legally inseparable.

Links

 * [1] What is Copyleft Definition by the Free Software Foundation
 * Copyleft Wikipedia definition