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Software License

A Software License is a legal instrument governing the use or redistribution of software. Under United States copyright law, all software is copyright protected, in both source code and object code forms, unless that software was developed by the United States Government, in which case it cannot be copyrighted. Authors of copyrighted software can donate their software to the public domain, in which case it is also not covered by copyright and, as a result, cannot be licensed.
wikipedia:: Software license

When you make a creative work (such as writing, graphics, or code), that work is under exclusive copyright by default. That is, the law assumes that as the author of your work, you have a say in what others can do with it. In general, that means nobody else can use, copy, distribute, or modify your work without being at risk of take-downs, shake-downs, or litigation. Open source is an unusual circumstance, however, because the author expects that others will use, modify, and share the work. But because the legal default is still exclusive copyright, you need a license that explicitly states these permissions. If you don’t apply an open source license, everybody who contributes to your project also becomes an exclusive copyright holder of their work. That means nobody can use, copy, distribute, or modify their contributions – and that “nobody” includes you.

Free Licenses, Copyleft

Software License
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Software License
MIT license
Apache 2.0 license
Free Licenses, Copyleft
GPL
Creative Commons - Creative Commons license