What is copyleft?
“Copyleft” open source licenses are, along with so-called “academic” licenses, one of the two main groups of open source licenses. “Copyleft” is a term intended to contrast with copyright, a legal right of ownership that authors of software usually use to maintain a monopoly in their works. In contrast, copyleft is a mechanism by which the owners of the code seek to use their ownership rights to ensure that not only will their code remain freely available to all users, but that any modifications and derivative works of the code made by those users will also be freely available on the same terms. In other words, owners of “copyleft” software attempt to use their copyright not to establish a monopoly, but to ensure that no one will ever be able to create proprietary rights based on their code. The Gnu Project’s General Public License (also known as the GPL) is probably the best-known and most widely used copyleft license.
