Open-source software (OSS) is computer software whose source code is publicly available, allowing users to use, study, modify, and distribute it — in contrast with proprietary (closed-source) software. These abilities are typically granted through open licenses or, more rarely, by dedication to the public domain. The development of open-source software can be based on open collaboration, a decentralized model of production. It is commonly defined according to the Open Source Definition of the Open Source Initiative, but the precise meaning of open source remains a source of conflicts.
Open-source software offers a range of benefits, but also presents specific risks and challenges. It can provide improved accessibility through lower and shared costs, greater adaptability through customization, transparency with auditable code, better interoperability via open standards, and independence from vendor lock-in, among others. Due to its ubiquity in modern software, open-source software raises major cybersecurity concerns, as well as significant maintenance challenges.
Open-source software has become the backbone of the global digital infrastructure, powering domains such as web servers, cloud computing, supercomputers, mobile devices, artificial intelligence, and Internet of Things. A 2022 estimate suggests that between 80 and 96 percent of the code making up software on the market today, including proprietary software, is of open-source origin. Its adoption has grown steadily across public institutions, the private sector, and academia, driven by motivations of cybersecurity, digital sovereignty, and independence from proprietary software, with significant geopolitical implications. Organizations are increasingly setting up Open Source Program Offices to manage their strategies.
Open-source software has its roots in the culture of source code sharing that dates back to the origins of computing in the 1950s. The term was formalized in 1998 with the founding of the Open Source Initiative, succeeding the free software movement initiated in the 1980s, with the intention of overcoming the ambiguity of the word "free" — meaning both freedom and gratis — to better appeal to the business world.
Open source software is mainly defined according to the Open Source Definition (OSD) established by the Open Source Initiative (OSI). To qualify as open source under this framework, software must provide its source code and be distributed under a license meeting the ten OSD criteria, notably including non-discrimination against persons, groups, or fields of endeavor — enabling anyone to use, modify, and redistribute it for any purpose. The definition was based on the Debian Free Software Guidelines, written and adapted primarily by Bruce Perens and Eric S. Raymond.. The OSI maintains a list of approved licenses that conform to the OSD.
However, the OSI holds no legal authority over the term 'open source' and its definition is a matter of social convention. With its non-discrimination criteria having generated for decades what is described as a 'culture war' within software communities over the very meaning of open source — the OSI can be seen by some actors as an oppressive dogma. It occurs because developers have never learned about this history, or don’t care about it, or actively think it’s irrelevant, involving a form of intergenerational conflict. These disputes are driven by ethical and commercial concerns. As a result, the term 'open source' can be used in ways that do not conform to the OSD, ranging from unintentional misuse to deliberate rejection of its authority.
Public-domain software is a source of disagreement as to whether it qualifies as open source software, carrying no formal licence and granting rights that vary significantly depending on the jurisdiction. The OSI itself has expressed diverging views on the matter, claiming both that "it is accurate to say that such software is effectively open source" and "it is incorrect to treat public domain software as open source".
Diverging visions and semantic conflicts around the meaning of open source have led to the creation of adjacent terms, such as source-available software, fair source, ethical source, shared source, post open source or fauxpen source.
"Open" versus "free" versus "free and open"
Free and open-source software (FOSS) or free/libre and open-source software (FLOSS) is openly shared source code that is licensed without any restrictions on usage, modification, or distribution. Confusion persists about this definition because the "free", also known as "libre", refers to the freedom of the product, not the price, expense, cost, or charge. For example, "being free to speak" is not the same as "free beer".
Conversely, Richard Stallman argues the "obvious meaning" of term "open source" is that the source code is public/accessible for inspection, without necessarily any other rights granted, although the proponents of the term say the conditions in the Open Source Definition must be fulfilled.
"Free and open" should not be confused with public ownership (state ownership), deprivatization (nationalization), anti-privatization (anti-corporate activism), or transparent behavior.
Gratis versus libre (no cost vs no restriction)
According to Feller et al. (2005), the terms "free software" and "open-source software" should be applied to any "software products distributed under terms that allow users" to use, modify, and redistribute the 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."
Despite initially accepting it, Richard Stallman of the FSF now flatly opposes the term "Open Source" being applied to what they refer to as "free software". Although he agrees that the two terms describe "almost the same category of software", Stallman considers equating the terms incorrect and misleading. Stallman also opposes the professed pragmatism of the Open Source Initiative, as he fears that the free software ideals of freedom and community are threatened by compromising on the FSF's idealistic standards for software freedom. The FSF considers free software to be a subset of open-source software, and Richard Stallman explained that DRM software, for example, can be developed as open source, despite that it does not give its users freedom (it restricts them), and thus does not qualify as free software.
Open-source software is owned by its authors who retain the copyright, from which they confer the right to use, modify and redistribute the software through the use of license. Without external agreements, contributions are owned by their authors resulting in copyright being shared among contributors. In this legal context, some projects use Contributor License Agreement (CLA) or a copyright assignment agreement to keep control over the copyright of the project, providing the ability to relicense the software under different terms. Some types of open source software are therefore described as proprietary software.
Open-source software development