The term "copyleft" was invented in 1984 by Richard Stallman, a
computer scientist at MIT (Massachusetts Institute of Technology), who
launched the GNU Project. As explained on its website: "Copyleft is a
general method for making a program or other work free, and requiring
all modified and extended versions of the program to be free as well.
(...) Copyleft says that anyone who redistributes the software, with or
without changes, must pass along the freedom to further copy and change
it. Copyleft guarantees that every user has freedom. (...) Copyleft is
a way of using of the copyright on the program. It doesn't mean
abandoning the copyright; in fact, doing so would make copyleft
impossible. The word 'left' in 'copyleft' is not a reference to the
verb 'to leave' - only to the direction which is the inverse of
'right'. (...) The GNU Free Documentation License (FDL) is a form of
copyleft intended for use on a manual, textbook or other document to
assure everyone the effective freedom to copy and redistribute it, with
or without modifications, either commercially or non commercially."