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What is FOSS?

FOSS is a umbrella initialism for Free and Open Source Software. It attempts to bring together two rival yet closely related factions within the world of software development: The Free Software movement, and the Open Source software community.

Generally speaking, both sides try to achieve the same sort of thing: Getting software source code into people's hands so they can study, share, modify, and use it as needed, give or take the caveats and requirements of whichever FOSS license is in play. The Free and Open Source movements each offer various licenses that define to varying degrees how free a particular piece of software can be. And that's free as in free speech as well as free as in free beer. You can be free to change some code, and give it away for free, for example.

The Free Software movement was founded by Richard Stallman in 1985 with the overall goal of giving people the freedom to use, improve, and distribute software without impeding the freedoms of others with additional restrictions and red tape. You're free to do what you want with the code, and even hand it out free of charge, but you have to, for instance, make available changes made to that code when necessary.

The Open Source software movement was formed in the 1990s by a collection of technologists to, essentially, make the concept of Free Software more palatable to businesses, downplaying the free-of-charge aspect and emphasizing that there is money to be made. Crucially, if you were to draw a Venn diagram, there would be a large circle representing all Open Source licenses, and inside that circle would be a smaller circle of copyleft licenses. The Open Source side is happy with Free Software licenses, but the Free Software world isn't happy with all Open Source licenses, particularly the permissive ones that allow arbitrary restrictions to be placed on code.

The Free Software camp thus does not like the newer term Open Source, especially when describing copyleft software, hence, the industry's use of FOSS, which encompasses both ideologies without implying they are one and the same.

FOSS is pronounced as the single-syllable word "foss" – there is no need to pronounce the letters, "eff-oh-ess-ess."