Free Software (DEF)

Main articles: The Free Software Definition, Debian Free Software Guidelines, and Open Source Definition

The first formal definition of free software was published by FSF in February 1986.
That definition, written by Richard Stallman, is still maintained today
and states that software is free software if people who receive a copy
of the software have the following four freedoms:

• Freedom 0: The freedom to run the program for any purpose.
• Freedom 1: The freedom to study and modify the program.
• Freedom 2: The freedom to copy the program so you can help your neighbor.
• Freedom 3: The freedom to improve the program, and release your
  improvements to the public, so that the whole community benefits.

Freedoms 1 and 3 require source code to be available because studying and modifying software without its source code is highly impractical.
Thus, free software means that computer users
have the freedom to cooperate with whom they choose, and to control the
software they use. To summarize this into a remark distinguishing libre (freedom) software from gratis (zero price) software, Richard Stallman said: "Free software is a matter of liberty, not price. To understand the concept, you should think of 'free' as in 'free speech', not as in 'free beer'".
In the late 90s, other groups published their own definitions which
describe an almost identical set of software. The most notable are Debian Free Software Guidelines published in 1997, and the Open Source Definition, published in 1998.

The BSD-based operating systems, such as FreeBSD, OpenBSD, and NetBSD,
do not have their own formal definitions of free software. Users of
these systems generally find the same set of software to be acceptable,
but sometimes see copyleft as restrictive. They generally advocate permissive free software licenses,
which allow others to make software based on their source code, and
then release the modified result as proprietary software. Their view is
that this permissive approach is more free. The Kerberos, X.org, and Apache
software licenses are substantially similar in intent and
implementation. All of these software packages originated in academic
institutions interested in wide technology transfer (University of California, MIT, and UIUC).