Copyleft
Copyleft is a legal concept and licensing approach that uses copyright law to ensure that derivative works of a software or creative work remain freely available under the same terms as the original. It is most commonly associated with free and open-source software (FOSS), where it requires that any modified or extended versions of the work be distributed under the same license, thus preserving user freedoms. This contrasts with permissive licenses that allow proprietary reuse.
Developers should understand and use copyleft when they want to create software that guarantees ongoing freedom for users and contributors, preventing proprietary appropriation. It is particularly valuable for community-driven projects, foundational libraries, or tools where widespread adoption and collaboration are priorities, such as in the Linux kernel (GPL) or GNU projects. However, it may be less suitable for commercial products seeking proprietary integration.