Definition - What does Copyleft mean?
Copyleft is free software license requiring copyright authors to permit some of their work to be reproduced. With copyright law, authors have complete control over their materials. But with copyleft law, users and authors co-exist. Users are permitted to engage in copying and distributing copyrighted materials. However, authors do have some say in who uses the materials based on their intended use. Copyleft does not require source code distribution. Thus, copyleft grants users similar rights to those normally only granted to the copyright authors, including activities such as distribution and copying.
Techopedia explains Copyleft
In the mid-1980s, Don Stallman coined the term copyleft in a letter he sent to Richard Stallman. Prior to the letter, Stallman created the very first copyleft when working on a Lisp interpreter. During that time, the Symbolics company asked permission to use it and eventually fine-tuned it after Stallman had provided them with a public domain of his work. Eventually, Stallman asked Symbolics for permission to review their improvements of Lisp, but Symbolics turned down his request. Disgruntled, Stallman set out to halt this type of quickly emerging proprietary software behavior by Emacs General Public License, which in 1984 was the original copyleft license. As time went on, this was renamed to the GNU General Public License.
Copyleft laws have provided users the same rights as copyright authors. They not only can review materials protected by copyright laws, but they can also copy, modify and distribute the materials. This gives many the benefit of using copyright materials, a “share all” type of use. However, to use copyleft, it must be determined (usually by the copyright author) that the materials are going to be used in a pertinent manner benefiting others for educational or cultural purposes.