A method and philosophy for software licensing and distribution designed to encourage use and improvement of software written by volunteers by ensuring that anyone can copy the source code and modify it freely.
The term "open source" is now more widely used than the earlier term "free software" (promoted by the Free Software Foundation) but has broadly the same meaning - free of distribution restrictions, not necessarily free of charge.
There are various open source licenses available. Programmers can choose an appropriate license to use when distributing their programs.
The Open Source Initiative promotes the Open Source Definition.
The Cathedral and the Bazaar. was a seminal paper describing the open source phenomenon.
Open Sources - O'Reilly book with full text online.