1.2.2. The Debian Free Software Guidelines
This reference document defines which software is “free enough” to be included in Debian. If a program's license is in accordance with these principles, it can be included in the main section; on the contrary, and provided that free distribution is permitted, it may be found in the non-free section. The non-free section is not officially part of Debian; it is an added service provided to users.
More than a selection criteria for Debian, this text has become an authority on the subject of free software, and has served as the basis for the “Open Source Definition”. Historically, it is therefore one of the first formal definitions of the concept of “free software”.