Copyleft

Copyleft is a play on the word copyright to describe the practice of using copyright law to offer the right to distribute copies and modified versions of a work and requiring that the same rights be preserved in modified versions of the work. In other words, copyleft is a general method for making a program (or other work) free (libre), and requiring all modified and extended versions of the program to be free as well. This free does not necessarily mean free of cost (gratis), but free as in freely available to be modified.