What is Free Software: Richard Stallman (2001)

“Free software” means software that respects users' freedom and community.

Roughly, it means that the users have the freedom to run, copy, distribute, study, change and improve the software.

Thus, “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”....

A program is free software if the program's users have the four essential freedoms:

The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.

The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.