The MIT License (also known as the X11 license or MIT X license) is a software license that was originally developed at the Massachusetts Institute of Technology. It is similar to the BSD license, which was first used for the Berkeley Source Distribution, a version of UNIX that was developed at the University of California at Berkeley (UCB). The main difference is that BSD-style licenses sometimes contain a clause prohibiting the use of the name of the copyright holder in promotions without permission.
Both the MIT and BSD licenses are considered to be more liberal than the GNU Public License (GPL), which is by far the most frequently used free software license. The reason for this is that the GPL requires that (1) all software derived from GPL-licensed software must also be released under the GPL license and (2) all re-distributions of GPL-licensed software, including modified versions or software derived from GPL-licensed software, must make the full source code freely available.
According to the Free Software Foundation (FSF), the MIT license should be referred to as the X11 license, because MIT has released software under a number of other licenses as well. (X11 is the current version of the X Window System, the de facto standard graphical engine for Linux and other Unix-like operating systems. It was developed at MIT.) The FSF is a non-profit organization that raises funds for work on the GNU project, one of the most notable products of which has been the GPL. However, the Open Source Initiative refers to it as the MIT License, as do many other groups. It should be kept in mind that the FSF is the same organization that insists that Linux should be renamed GNU/Linux (because of the large amount of GNU software included in Linux distributions). A good compromise is to refer to it as the MIT X License.
There is also some disagreement as to which type of license does a better job of protecting end users' rights. The MIT license more explicitly states the rights given to users, 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 . . ." However, advocates of the GPL claim that its requirements about derived software and providing source code do a better job of protecting users.
The MIT license is used for a number of software packages, including expat (an XML 1.0 parser written in C), MetaKit (an efficient embedded database library with a small footprint), Open For Business Project (provides tools and applications for business based on Sun's J2EE standard), X11 and XFree86 (an open source implementation of X86).
Because the MIT License is not copyrighted, in contrast to the GPL, other developers are free to modify it to suit their own requirements. For example, the Free Software Foundation employs a license identical to the MIT License for its ncurses library, except for the addition of this text:
Except as contained in this notice, the name(s) of the above copyright holders shall not be used in advertising or otherwise to promote the sale, use or other dealings in this Software without prior written authorization.
(Ncurses is a library of functions that manages an application's display on character-cell terminals. It is a part of the GNU project, but it is one of the few GNU programs that is not released under the GPL.)
The basic template of the MIT License is as follows:
Copyright (c) <year> <copyright holders>
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 above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
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.
* Disclaimer: The above material is presented for reference purposes only, and it is not intended as nor does it constitute legal advice. This definition was reprinted with permission from The Linux Information Project (LINFO).