The MIT License is a free software license originating at the Massachusetts Institute of Technology (MIT). It is a permissive free software license, meaning that it permits reuse within proprietary software provided all copies of the licensed software include a copy of the MIT License terms. Such proprietary software retains its proprietary nature even though it incorporates software under the MIT License. The license is also GPL-compatible, meaning that the GPL permits combination and redistribution with software that uses the MIT License.

Software packages that use one of the versions of the MIT License include Expat, PuTTY, the Mono development platform class libraries, Ruby on Rails, CakePHP, Symfony, Lua (from version 5.0 onwards), and the X Window System, for which the license was written.

Because MIT has used many licenses for software, "MIT License" is considered ambiguous by the Free Software Foundation. "MIT License" may refer to the "Expat License" (used for Expat) or to the "X11 License" (also called "MIT/X Consortium License"; used for the X Window System by the MIT X Consortium). The "MIT License" published on the official site of Open Source Initiative is the same as the "Expat License".