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The terms “free software” and “open source” are the slogans of two different movements which differ in their basic philosophy. The Free Software Movement is idealistic, and raises issues of freedom, ethics, principle and what makes for a good society. The Open Source Movement, founded in 1998, studiously avoids such questions. For more explanation, see http://www.gnu.org/philosophy/free-software-for-freedom.html.
The GNU Project is aligned with the Free Software Movement. This doesn't mean that all GNU contributors and maintainers have to agree; your views on these issues are up to you, and you're entitled to express them when speaking for yourself.
However, due to the much greater publicity that the Open Source Movement receives, the GNU Project needs to overcome a widespread mistaken impression that GNU is and always was an activity of the Open Source Movement. For this reason, please use the term “free software,” not “open source,” in GNU software releases, GNU documentation, and announcements and articles that you publish in your role as the maintainer of a GNU package. A reference to the URL given above, to explain the difference, is a useful thing to include as well.