Copyleft ?

At first sight, we might think that the notion of "copyleft" is just the opposite of the notion of copyright (and a play on the word copyright). But "copyleft" more describes the possibility to use, study, distribute, copy or alter someone's work with the permission from the creation's author. The only restriction is that modified versions of a document can be modify as well. "Copyleft" is a different form of licensing and the main goal of this type of license is to offer free available content.

To go further, here is a text from Richard Stallman, who developped one of the first copyleft license called "GNU General Public License": http://www.gnu.org/philosophy/pragmatic.html

The "reversed" C is the copyleft symbol.