Generally speaking, free software license is a phrase used by the free software movement to mean any software license that meets the free software definition of the Free Software Foundation (FSF). This definition refers to the four kinds of freedom for users of software:

0. The freedom to run the program for any purpose
1. The freedom to study and modify the program
2. The freedom to copy the program
3. The freedom to redistribute modified versions of the program

(Their numbering is zero-based in traditional hacker style.) A license that additionally requires that those freedoms be preserved for modified works is a copyleft license. See Free software movement for more information. The Free Software Foundation maintains a list of free software licenses at their web site. The list distinguishes between free software licenses that are compatible or incompatible with the FSF license of choice, the GNU General Public License, which is a copyleft license. The list also contains licenses which the FSF considers non-free for various reasons. Note that the open source license list differs slightly, but in almost all cases the definitions apply to the same licenses. The Debian Free Software Guidelines are also frequently used to determine whether a license is a free software license.

External links

• The Free Software Definition (Free Software Foundation, 2004).
• The Free Software Foundation's list of free and unfree licenses