A type of license recognized by the The Open Source Initiative (OSI) that complies with the Open Source Definition. The MIT license permits reuse within proprietary software on the condition that the license is distributed with that software. The MIT license is one of well over fifty licenses that have been approved by the OSI. Open Source Initiative OSI. Ruby on Rails is one of many well-known software packages that is distributed under the MIT license.
See "What is Open Source Software?" for more information.