Open source or open-source is the idea that source code for a piece of software is freely available for viewing and modifying. Many open source licences are approved by the Open Source Initiative. Open source licences approved by the Open Source Initiative may be referred to as OSI approved licences.
It should be noted that while the term "open source" has gained wide-spread acceptance, there are still those who reject it. The most visible example is Richard Stallman, founder of the GNU Project and author of the GPL and several widely-used software packages, who instead continues to use the older term free software. (For a brief discussion of his reasons, see the Terminology section of his Wikipedia entry.) The possible confusion of the word "free" prompted Stallman to explain it this way:
"Free software" is a matter of liberty, not price. To understand the concept, you should think of "free" as in "free speech," not as in "free beer."