A copyright notice in which the rights for use, modification, and reproduction of the product are granted to any and all users.
To copyleft a program, first we copyright it; then we add distribution terms, which are a legal instrument that gives everyone the rights to use, modify, and redistribute the program's code or any program derived from it but only if the distribution terms are unchanged. Thus, the code and the freedoms become legally inseparable.
Proprietary software developers use copyright to take away the users' freedom; we use copyright to guarantee their freedom. That's why we reverse the name, changing 'copyright' into 'copyleft.'
Richard Stallman, The GNU Project
GNU software is freely distributed, but in a different manner from public domain and ''freeware'' software among personal computer users. While public domain software can be freely copied, freeware authors ask users to contribute a fee if they find a program useful. In contrast, GNU programs are not placed in the public domain. Instead they are distributed with a public license that Mr. Stallman calls a ''copyleft.'' This license insures that the software will stay freely copyable and not be incorporated into a for-profit program.
John Markoff, "One Man's Fight for Free Software," The New York Times, January 11, 1989