Copyleft
Copyleft is a licensing strategy that uses copyright law to require derivative works to be distributed under the same permissive terms as the original. Coined in the free-software movement and operationalized by the GNU GPL, it has since spread to content licensing through Creative Commons ShareAlike and to databases through the ODbL.
Copyleft is a licensing strategy that flips copyright on its head: instead of reserving rights, the author grants broad permissions on the condition that anyone redistributing the work, or works derived from it, must do so under the same permissive terms. The mechanism is enforced through ordinary copyright law, so a downstream redistributor who strips the license is committing infringement. The term was popularized in the 1980s in the free-software movement around Richard Stallman and the Free Software Foundation, and it is most strongly associated with the GNU General Public License. The GPL requires that any program that links to or incorporates GPL code be itself released under the GPL when distributed; see The GNU General Public License (GPL): How Copyleft Works for the software case. The Affero GPL extends the obligation to network-served software. Copyleft is divided informally into "strong" and "weak" variants. Strong copyleft (GPL, AGPL) extends to anything that incorporates the licensed code. Weak copyleft (LGPL, MPL) limits the obligation to changes in the licensed files themselves, allowing larger proprietary works to link to copyleft components without becoming copyleft. The concept migrated beyond software. The ShareAlike element in Creative Commons Licensing applies copyleft to text, images, and other media: CC BY-SA content can be remixed freely, but the resulting work must be released under a compatible SA license. The Open Database License (ODbL), used by OpenStreetMap, extends a similar mechanism to databases. The GNU Free Documentation License was an earlier copyleft license for documentation, used by Wikipedia until its 2009 migration to CC BY-SA. Copyleft is contentious. Proponents argue it keeps a commons from being privately enclosed; critics argue the viral obligation deters commercial adoption and is incompatible with permissive licenses like MIT or BSD, fragmenting the open ecosystem. In practice many large projects adopt permissive licenses precisely to avoid the compliance complexity of strong copyleft.