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, and requiring all modified and extended versions of the program to be free as well. It is a a form of licensing and can be used to maintain copyright conditions for works such as computer software, documents, music and art.
The Daily Omnivore 
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