"Free and open-source software" (FOSS) is an umbrella term for software that is simultaneously considered both Free software and open-source software. FOSS (free and open-source software) allows the user to inspect the source code and provides a high level of control of the software's functions compared to proprietary software. The term "free software" does not refer to the monetary cost of the software at all, but rather whether the license maintains the software user's civil liberties ("free” as in “free speech,” not as in “free beer”). There are a number of related terms and abbreviations for free and open-source software (FOSS or F/OSS), or free/libre and open-source software (FLOSS or F/LOSS—FLOSS is the FSF-preferred term).
Although there is almost a complete overlap between free-software licenses and open-source-software licenses, there is a strong philosophical disagreement between the advocates of these two positions. The terminology of FOSS or "Free and Open-source software" was created to be a neutral on these philosophical disagreements between the FSF and OSI and have a single unified term that could refer to both concepts.
As the Free Software Foundation (FSF) explains the philosophical difference between free software and open-source software: "The two terms describe almost the same category of software, but they stand for views based on fundamentally different values. Open-source is a development methodology; free software is a social movement. For the free-software movement, free software is an ethical imperative, essential respect for the users' freedom. By contrast, the philosophy of open-source considers issues in terms of how to make software “better”—in a practical sense only." In parallel to this the Open Source Initiative (OSI) considers many free-software licenses to also be open source. These include the latest versions of the FSF's three main licenses: the GPL, the Lesser General Public License (LGPL), and the GNU Affero General Public License (AGPL).
Richard Stallman's Free Software Definition, adopted by the Free Software Foundation (FSF), defines free software as a matter of liberty not price, and it upholds the Four Essential Freedoms. The earliest-known publication of the definition of his free-software idea was in the February 1986 edition of the FSF's now-discontinued GNU's Bulletin publication. The canonical source for the document is in the philosophy section of the GNU Project website. As of August 2017, it is published there in 40 languages.
Four essential freedoms of Free Software
To meet the definition of "free software", the FSF requires the software's licensing respect the civil liberties / human rights of what the FSF calls the software user's "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.
The open-source-software definition is used by the Open Source Initiative (OSI) to determine whether a software license qualifies for the organization's insignia for Open-source software. The definition was based on the Debian Free Software Guidelines, written and adapted primarily by Bruce Perens. Perens did not base his writing on the Four Essential Freedoms of free software from the Free Software Foundation, which were only later available on the web. Perens subsequently stated that he felt Eric Raymond's promotion of Open-source unfairly overshadowed the Free Software Foundation's efforts and reaffirmed his support for Free software. In the following 2000s, he spoke about open source again.