The free software movement was conceived in 1983 by Richard Stallman to give the benefit of "software freedom" to computer users. From the late 1990s onward, alternative terms for free software came into use. The most common are "software libre", "free and open source software" ("FOSS") and "free, libre and open-source software" ("FLOSS"). The "Software Freedom Law Center" was founded in 2005 to protect and advance FLOSS. The antonym of free software is "proprietary software" or "non-free software".
The "four essential freedoms" which determines if software is free or non-free
Free software refers to software which gives end-users the right to use, study, modify and distribute the software without restrictions. It is not the same as "freeware" - a term which refers to software binaries which are available at zero price. The free in free software refers to freedom, not price.
The Free Software Definition, as defined by Richard Stallman, is
"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 others (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."
Free software vs Open Source
The terms "free software" and "open source" are by many used interchangeably to describe the same thing but they do not describe the same concept.
"Free software" means that software is both free of charge, free to use, free to change and free to re-distribute. It is a term that implies that the users of such software have essential freedoms.
"Open source" can simply mean that the source-code is available for study. It does not mean that it is legal to redistribute binaries or modified copies of the source.
As Olivier Cleynen points out in the lecture Overtaking Proprietary Software Without Writing Code: Using the term "Free software" makes it clear that people's freedom is at stake.