Free Software Foundation
The Four Essential Freedoms
A program is free software if the program's users have the four essential freedoms:
◾The freedom to run the program as you wish, for any purpose (freedom 0).
◾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 redistribute copies so you can help your neighbor (freedom 2).
◾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.