The MIT License is one of the shortest and most permissive open-source licenses. It lets anyone use, copy, modify, merge, publish, distribute, sublicense, and sell copies of the software, provided they include the original copyright notice and license text.
It is a good default for libraries and frameworks you want adopted as widely as possible. If you care about protecting against patent claims from contributors, consider Apache-2.0 instead — the patent grant is the main difference.
How to apply
Add a file named LICENSE in the root of your project containing the full MIT license text, and make sure the copyright line names the right author and year. No per-file header is required, but many projects add a short SPDX identifier at the top of each file:
// SPDX-License-Identifier: MIT
Full license text
MIT License
Copyright (c) [year] [fullname]
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.
Raw text also available at /license-texts/MIT.txt — handy for curl.