The Free Software Definition

The Free Software Definition written by Richard Stallman and published by Free Software Foundation, defines free software as being software that ensures that the end users have freedom in using, studying, sharing and modifying that software. The term "free" is used in the sense of "free speech," not of "free of charge." The earliest-known publication of the definition was in the February 1986 edition of the now-discontinued GNU's Bulletin publication of FSF. The canonical source for the document is in the philosophy section of the GNU Project website., it is published there in 39 languages. FSF publishes a list of licences which meet this definition.

The definition and the Four Freedoms

The definition published by FSF in February 1986 had two points:
In 1996, when the website was launched, "free software" was defined referring to "three levels of freedom" by adding an explicit mention of the freedom to study the software. Stallman later avoided the word "levels", saying that you need all of the freedoms, so it's misleading to think in terms of levels.
Finally, another freedom was added, to explicitly say that users should be able to run the program. The existing freedoms were already numbered one to three, but this freedom should come before the others, so it was added as "freedom zero".
The modern definition defines free software by whether or not the recipient has the following four freedoms:
Freedoms 1 and 3 require source code to be available because studying and modifying software without its source code is highly impractical.

Later definitions

In July 1997, Bruce Perens published the Debian Free Software Guidelines. A definition based on the DFSG was also used by the Open Source Initiative under the name "The Open Source Definition".