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An open source license is a type of license for computer software and other products that allows the source code, blueprint or design to be used, modified and/or shared under defined terms and conditions. This allows end users to review and modify the source code, blueprint or design for their own customization, curiosity or troubleshooting needs. Open-source licensed software is mostly available free of charge, though this does not necessarily have to be the case. Licenses which only permit non-commercial redistribution or modification of the source code for personal use only are generally not considered as open-source licenses. However, open-source licenses may have some restrictions, particularly regarding the expression of respect to the origin of software, such as a requirement to preserve the name of the authors and a copyright statement within the code, or a requirement to redistribute the licensed software only under the same license (as in a copyleft license). One popular set of open source software licenses are those approved by the Open Source Initiative (OSI) based on their Open Source Definition (OSD).
The Free Software Foundation has a related but distinct critera for evaluating whether or not a license qualifies software as free software. All free software licenses are also considered open-source software licenses. In the same way, the Debian project has its own criteria, the Debian Free Software Guidelines, on which the Open Source Definition is based. Open-source license criteria focuses on the availability of the source code and the ability to modify and share it, while free software licenses focuses on the users freedom to use the program, to modify it, and to share it.
There are also shared source licenses which have some similarities with open source, such as the Microsoft Reciprocal License (MS-RL). They are mainly used by Microsoft and can range from extremely restrictive to comparable with free open-source software.