The MIT License is a very permissive, non-copyleft software license. Most remarkable about it is its shortness. The first time I came upon an MIT license I had to read it a few times to ensure that was all there was to it. (Something quite unheard of with most licenses, multiple pages in length, such as the GPL.) It is nearly identical to the BSD license.

In essence, the MIT license disavows most of one's monopolies granted under copyright and has a standard disclaimer of liability. It requires that the license itself be included with copies but does not make any restrictions on derivatives beyond that. One can modify code distributed under the MIT license, sell it, and not provide the source for the original. The term "MIT license"1 can be misleading. X11 has kept an MIT-style license since its inception (at MIT) which has stayed with it as it's passed hands multiple times to its current status with X.org. But the X11 license (or X Consortium license as it's sometimes called) includes an advertising restriction which makes it nearly identical to the BSD license.

The unambiguous name for the MIT license, sans advertising restrictions, is the Expat license. Unfortunately, this identification is only unambiguous if one has a short memory. Prior to its current status4, Expat was actually under the Mozilla Public License Version 1.0, a much longer document5. Just the same, "expat license" appears to be the preferred name for the MIT license due to the various other licenses which also came directly from MIT.

The full text of the MIT license is reproduced below:2

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

References:

  1. "Various Licenses and Comments about Them"
    http://www.gnu.org/philosophy/license-list.html
  2. "The MIT License:Licensing"
    http://www.opensource.org/licenses/mit-license.php
  3. "Quick Reference For Choosing a Free Software License"
    http://zooko.com/license_quick_ref.html
  4. Expat License via CVS
    http://expat.cvs.sourceforge.net/expat/expat/COPYING?revision=1.2&view=markup
  5. "expat"
    http://www.xml.com/pub/r/774

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