The Jargon File (version 4.4.7, 29 Dec 2003):

copyleft
 /kop'ee?left/, n.

    [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 (30 December 2018):

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.

   [Jargon File]

   (1995-04-18)