Copyleft is a play on the word copyright and describes the practice of using copyright law to remove restrictions on distributing copies and modified versions of a work for others and requiring that the same freedoms be preserved in modified versions.The GNU Project goes on to answer the question "what is copyleft?" with the answer:
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.The Free Software Foundation expands on the topic a bit, saying
To copyleft a program, we first state that it is copyrighted; 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.''
So, now you know. I want to thank the student for challenging me on the definition. Asking about the veracity and validity of things you hear and read is what this class is all about and that was a good example being a skeptical consumer of information.