Open source
Open source or open-source is the idea that source code for a piece of software is freely available for viewing and modifying. The term "Open Source" is a trademark of the Open Source Initiative; This means that OSI must approve of a license before it can be described as an open-source license.
The MIT/X11 License and the GPL are examples of open source licenses.
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."