Free and open-source software
Free and open-source software (FOSS) is software available under a license that grants users the right to use, modify, and distribute the software – modified or not – to everyone. FOSS is an inclusive umbrella term encompassing free software and open-source software. The rights guaranteed by FOSS originate from the "Four Essential Freedoms" of The Free Software Definition and the criteria of The Open Source Definition. All FOSS can have publicly available source code, but not all source-available software is FOSS. FOSS is the opposite of proprietary software, which is licensed restrictively or has undisclosed source code.
The historical precursor to FOSS was the hobbyist and academic public domain software ecosystem of the 1960s to 1980s. Free and open-source operating systems such as Linux distributions and descendants of BSD are widely used, powering millions of servers, desktops, smartphones, and other devices. Free-software licenses and open-source licenses have been adopted by many software packages. Reasons for using FOSS include decreased software costs, increased security against malware, stability, privacy, opportunities for educational usage, and giving users more control over their own hardware.
The free software movement and the open-source software movement are online social movements behind widespread production, adoption and promotion of FOSS, with the former preferring to use the equivalent term free/libre and open-source software (FLOSS). FOSS is supported by a loosely associated movement of multiple organizations, foundations, communities and individuals who share basic philosophical perspectives and collaborate practically, but may diverge in detail questions.
Overview
"Free and open-source software" (FOSS) is an umbrella term for software that is considered free software and open-source software. The precise definition of the terms "free software" and "open-source software" applies them to any software distributed under terms that allow users to use, modify, and redistribute said software in any manner they see fit, without requiring that they pay the author(s) of the software a royalty or fee for engaging in the listed activities.
Although there is an almost 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 was created to be a neutral on these philosophical disagreements between the Free Software Foundation (FSF) and Open Source Initiative (OSI) and have a single unified term that could refer to both concepts, although Richard Stallman argues that it fails to be neutral unlike the similar term; "Free/Libre and Open Source Software" (FLOSS).
History
In the early decades of computing, particularly from the 1950s through the 1970s, software development was largely collaborative. Programs were commonly shared in source code form among academics, researchers, and corporate developers. Most companies at the time made their revenue from hardware sales, and software—including source code—was distributed freely alongside it, often as public-domain software.
By the late 1960s and 1970s, a distinct software industry began to emerge. Companies started selling software as a separate product, leading to the use of restrictive licenses and technical measures—such as distributing only binary executables—to limit user access and control. This shift was driven by growing competition and the U.S. government's antitrust scrutiny of bundled software, exemplified by the 1969 antitrust case United States v. IBM.
A key turning point came in 1980 when U.S. copyright law was formally extended to cover computer software.[27][28] This enabled companies like IBM to further enforce closed-source distribution models. In 1983, IBM introduced its "object code only" policy, ceasing the distribution of source code for its system software.
In response to the growing restrictions on software, Richard Stallman launched the GNU Project in 1983 at MIT. His goal was to develop a complete Free software operating system and restore user freedom. The Free Software Foundation (FSF) was established in 1985 to support this mission. Stallman's GNU Manifesto and the Four Essential Freedoms outlined the movement's ethical stance, emphasizing user control over software.
The release of the Linux kernel by Linus Torvalds in 1991, and its relicense under the GNU General Public License (GPL) in 1992, marked a major step toward a fully Free operating system. Other Free software projects like FreeBSD, NetBSD, and OpenBSD also gained traction following the resolution of the USL v. BSDi lawsuit in 1993.
In 1997, Eric Raymond’s essay The Cathedral and the Bazaar explored the development model of Free software, influencing Netscape’s decision in 1998 to release the source code for its browser suite. This code base became Mozilla Firefox and Thunderbird.
To broaden business adoption, a group of developers including Raymond, Bruce Perens, Tim O’Reilly, and Linus Torvalds rebranded the Free software movement as "Open Source." The Open Source Initiative (OSI) was founded in 1998 to promote this new term and emphasize collaborative development benefits over ideology.
Despite initial resistance—such as Microsoft's 2001 claim that "Open-source is an intellectual property destroyer"—FOSS eventually gained widespread acceptance in the corporate world. Companies like Red Hat proved that commercial success and Free software principles could coexist.