The Free Software Definition is the Free Software Foundation’s authoritative statement of what makes a program “free software.” Maintained at gnu.org/philosophy/free-sw, it is the reference text the movement points to when deciding whether a given license or program qualifies. Its first job is to dispel a common confusion: the word “free” here is about liberty, not cost. The page states that “‘Free software’ means software that respects users’ freedom and community” and that the issue “is a matter of liberty, not price,” summarized in the slogan that one should “think of ‘free’ as in ‘free speech,’ not as in ‘free beer.’”
From that premise the definition asserts a precise test: “a program is free software if the program’s users have the four essential freedoms.” These are numbered freedom 0 through freedom 3, covering the freedom to run the program for any purpose, to study and change it, to redistribute exact copies, and to distribute modified versions. A program that grants all four to its users is free software; one that withholds any of them is, to that extent, not.
The definition is careful about preconditions and edge cases. Access to source code is named as a precondition for the freedoms to study and to improve a program, since one cannot meaningfully change software whose source one cannot read. The text also explains that the freedom to run the program means running it “for any purpose,” without having to report to or seek permission from the developer or any other party.
The page has been revised many times since its first publication in the 1980s, and gnu.org keeps a history of those revisions, but the four-freedoms core has remained stable. Refinements have addressed questions such as whether a license may require modified versions to carry a different name, how distribution for a fee interacts with freedom, and what kinds of restrictions are acceptable without violating the freedoms. Each clarification is presented as an application of the same underlying principles rather than a change to them.
The Free Software Definition is the conceptual foundation beneath the GNU General Public License and the FSF’s evaluation of other licenses. Licenses are judged free or non-free by whether they preserve the four freedoms for all users, which makes this short document the philosophical bedrock of copyleft and of the free software movement as a whole.