Copyleft

Copyleft is a licensing strategy where a software license uses copyright law to ensure that derivative works also remain free and open source. Coined by Richard Stallman as a play on "copyright" — you keep the right reserved, but use it to mandate freedom rather than restrict it.

Strong copyleft licenses (GPL, AGPL) require all derivative works to use the same license. Weak copyleft licenses (LGPL, MPL) only require modifications to the original library to stay copyleft — surrounding application code can be proprietary.

Permissive licenses (MIT, Apache, BSD) are the opposite: they let derivative works be relicensed however the new author wants, including as fully proprietary.

The trade-off: copyleft maximizes freedom for downstream users (everyone gets access to derivatives) but reduces adoption (some companies can't or won't use copyleft code). Permissive licenses maximize adoption (anyone can use it) but allow downstream "extraction" (cloud providers selling managed services without sharing).