The MIT License is a free software license originating at the Massachusetts Institute of Technology (MIT). It is a permissive license, meaning that it permits reuse within proprietary software on the condition that the license is distributed with that software. The license is also GPL-compatible, meaning that the GPL permits combination and redistribution with software that uses the MIT License.
According to the Free Software Foundation, since MIT has used many licenses for software, "MIT License" is considered ambiguous, it may accurately refer to the Expat license (used for the Expat) or the X11 license (used for the X Window System by the MIT X Consortium). The "MIT License" often used and published on the official site of Open Source Initiative is the same as the Expat license.
Comparing to the Expat license, X11 license and the "MIT License" used for ncurses by the Free Software Foundation in 1998 adds this clause:
The XFree86 Project uses a modified MIT License for XFree86 version 4.4 onward. The license includes a clause that requires attribution in software documentation. The Free Software Foundation contends that this addition is incompatible with the version 2 of the GPL, but compatible with version 3:
Software packages that use one of the versions of the MIT License include Expat, PuTTY, the Mono development platform class libraries, Ruby on Rails, Lua (from version 5.0 onwards), and the X Window System, for which the license was written.
A common form of the MIT license (the version same as the Expat license, not identical with the X sources) is defined as follows:
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