|(legal)||copyleft - /kop'ee-left/ (A play on "copyright") The copyright
notice and General Public License applying to the works of
the Free Software Foundation, granting reuse and
reproduction rights to everyone.|
Typically copyrights take away freedoms; copyleft preserves them. It is a legal instrument that requires those who pass on a program to include the rights to use, modify, and redistribute the code; the code and the freedoms become legally inseparable.
The copyleft used by the GNU Project combines a regular copyright notice and the "GNU General Public License" (GPL). The GPL is a copying license which basically says that you have the aforementioned freedoms. The license is included in each GNU source code distribution and manual.
See also General Public Virus.