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".
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.