BSD许可证
Copyright licenses known as "open" or free licenses seek to grant several rights to licensees, either for a fee or not. "Free" in this context is not as much of a reference to price as it is to freedom. What constitutes free licensing has been characterised in a number of similar definitions, including by order of longevity the Free Software Definition, the Debian Free Software Guidelines, the Open Source Definition and the Definition of Free Cultural Works. Further refinements to these definitions have resulted in categories such as copyleft and permissive. Common examples of free licences are the GNU General Public License, BSD licenses and some Creative Commons licenses.