Copyleft
Copyleft is a category of open source licensing that requires anyone who distributes a modified or derivative version of the software to release it under the same license terms, keeping the code and its derivatives freely available.
Copyleft is a licensing approach that uses copyright law to guarantee that software and its derivatives remain open. Where a permissive license lets recipients do almost anything with minimal conditions, a copyleft license adds a reciprocal obligation: if you distribute a modified or derived version, you must license that version under the same terms and typically provide the source code.
Copyleft strength varies. Strong copyleft licenses extend their requirements to a broader combined work, while weak or file-level copyleft licenses limit reciprocity to the licensed files themselves, allowing them to be linked into otherwise differently licensed software. Some licenses also extend obligations to software offered over a network, not just software distributed as files.
For developers, understanding copyleft is essential because using a copyleft-licensed dependency can create obligations for the code that combines with it. Choosing dependencies with the appropriate copyleft strength, and knowing when distribution triggers obligations, is a key part of license compliance and protecting proprietary code.