Copyleftlisenser

Copyleft licenses, often grouped under the term copyleftlisenser in some contexts, are a family of free and open-source licenses that require derivative works to be distributed under the same or compatible copyleft terms. The core idea is to preserve the freedom to use, study, modify, and share software for all users, by ensuring that improvements remain free for subsequent redistributions.

Copyleft licenses are typically categorized as strong or weak. Strong copyleft licenses, such as the GNU General Public License (GPL) and GNU Affero General Public License (AGPL), require that any derivative work or redistribution of the modified software also be licensed under the same terms. This can extend to the entire combined work when the code is linked or incorporated. Weak copyleft licenses, such as the GNU Lesser General Public License (LGPL) and the Mozilla Public License (MPL), apply copyleft terms to the original files or modules, while allowing linking or combining with non-copyleft components under fewer restrictions. Permissive licenses (for comparison) like MIT, BSD, and Apache do not impose copyleft on derivative works.

In practice, copyleft requires distributors of binaries to provide access to source code and to include the license text and notices, ensuring downstream users can exercise the same freedoms. Network use can also be covered, as with AGPL, which closes certain loopholes where software runs remotely but remains isolated from distribution obligations. Copyleft licenses influence how software can be used in proprietary products, the design of software ecosystems, and collaboration strategies among developers and organizations.

Historically, copyleft emerged with the GNU Project in the late 1980s as a mechanism to guarantee software freedom. Today, they remain a core option for projects seeking to ensure that improvements stay open and that user rights are preserved in downstream distributions.