Copyleft
Copyleft is the Free-Software Foundations way of describing the operations of GPL licenses.
Copyleft is a legal concept derived from free-software that allows for the copying, distribution, and modification of works while prohibiting ownership.
The term "Copyleft" is a play on the word "copyright", intellectual property protection for the creators. According to the Free Software Foundations thinking, "Copyleft" uses copyright law to protect users instead of creators: anyone can use, modify, and re-distribute the modified software.