The Free Software Definition

The definition published by the FSF in 1986 originally listed two key points: Stallman later discouraged describing them as "levels," noting that all freedoms are equally essential. A fourth freedom was later introduced to explicitly affirm the user's right to run the program. Because it was seen as more fundamental than the others, it was placed first. Since the existing freedoms were numbered one through three, this new one was designated "freedom zero". Such zero-based numbering, where counting begins at zero, is also a common convention in computer programming. The modern definition states that a program is free software if it grants users the following four essential freedoms: Freedoms 1 and 3 require source code to be available, as studying and modifying software without source code is impractical. ==Later definitions==

In July 1997, Bruce Perens published the Debian Free Software Guidelines. A definition based on the DFSG was later adopted by the Open Source Initiative (OSI) under the name The Open Source Definition. ==Comparison with the Open Source Definition==

Despite philosophical differences between the free software movement and the open-source movement, the FSF's definition of free software and the OSI's definition of open-source software describe largely overlapping sets of licenses, with only a few minor differences. While emphasizing these philosophical distinctions, the FSF states: ==See also==