The Jargon File (version 4.4.7, 29 Dec 2003):
[play on copyright]
1. The copyright notice (?General Public License?) carried by GNU EMACS
and other Free Software Foundation software, granting reuse and
reproduction rights to all comers (but see also General Public Virus).
2. By extension, any copyright notice intended to achieve similar aims.
The Free On-line Dictionary of Computing (18 March 2015):
/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
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.