The Free Software Definition, written by Richard Stallman and published by Free Software Foundation (FSF), defines free software, as a matter of liberty, not price. The term "free" is used in the sense of "free speech," not of "free beer." 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. As of April 2008, it is published there in 39 languages. FSF publishes a list of licences which meet this definition.

It was by far the earliest published definition for the concept of free software.
The definition published by FSF in February 1986 had two points:

The word "free" in our name does not refer to price; it refers to freedom. First, the freedom to copy a program and redistribute it to your neighbors, so that they can use it as well as you. Second, the freedom to change a program, so that you can control it instead of it controlling you; for this, the source code must be made available to you.

The modern definition has four points, which it numbers zero to three. It defines free software by whether or not the recipient has the freedoms to:

- run the program, for any purpose (freedom 0)
- study how the program works, and adapt it to your needs (freedom 1)
- redistribute copies so you can help your neighbor (freedom 2)
- improve the program, and release your improvements to the public, so that the whole community benefits (freedom 3)

It also notes that "Access to the source code is a precondition" for freedoms 1 and 3.
In July 1997, Bruce Perens published the Debian Free Software Guidelines. This was also used by Open Source Initiative (OSI) under the name "The Open Source Definition", the only change being that use of the term "free software" was replaced by OSI's alternative term for free software, "open-source software".