An open source project is a project in which developers make their source code available publicly to end users. The term applies to software licensed under an open source license (as opposed to a proprietary license). The main purpose of making a program open source is so that users are free to redistribute it. Open source software is often developed non-commercially, although its development is sometimes supported by major corporations. Even though many open source programs are available at no cost, they are not required to be. The precise rules and specifications for an officially open source license are maintained by the Open Source Initiative (OSI) in the Open Source Definition available at http://www.opensource.org/docs/osd. Open source software is usually easily obtainable and can be freely modified--often with the expectation that the modified version be open source as well.