The MIT License is a short, permissive software license that grants nearly unrestricted rights to use a program in exchange for one obligation: preserving the copyright notice and the license text. The Open Source Initiative publishes its canonical wording, which opens “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.” That single sentence covers the full set of freedoms most permissive licenses spell out at greater length.
The only condition the license imposes is attribution. As the OSI text states, the rights are granted “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.” There is no copyleft requirement, no patent clause, and no obligation to publish source for derivative works, which is what distinguishes it from a copyleft license such as the GPL.
The remainder of the text is a warranty disclaimer, written in capital letters, declaring that the software is provided “AS IS, WITHOUT WARRANTY OF ANY KIND” and that the authors are not liable for damages. This three-part shape, a permission grant, an attribution condition, and a disclaimer, is the entire license; it fits comfortably on a single page and is short enough to read in under a minute.
The license is also known as the Expat License, because the form most often used today matches the wording adopted by the Expat XML parser library. The Software Package Data Exchange (SPDX) license list assigns it the identifier MIT with the full name “MIT License,” and that identifier is the value most projects place in a package manifest or a LICENSE file header.
The MIT License grew out of licensing practices at the Massachusetts Institute of Technology in the late 1980s, associated with software such as the X Window System, and its brevity and clarity helped it spread far beyond its origin. Because it permits proprietary reuse and asks almost nothing in return, it became a default choice for libraries, frameworks, and tooling across the software ecosystem, and surveys of package registries and code hosts have consistently found it to be the most common open source license in use.