What is FOSS (Free and Open-Source Software)?
Software whose license lets anyone use, study, change and redistribute it, with the source code available. FOSS covers both "free software" and "open source"; FLOSS and OSS are near-synonyms.
Definition
FOSS stands for free and open-source software: software released under a license that lets anyone run it for any purpose, read and modify the source code, and share copies, changed or unchanged. "Free" here means freedom, not price. A FOSS project can be sold, and plenty of free-of-charge software (freeware) is not FOSS because you can't see or change its code. The term joins two movements that agree on most licenses but describe them differently. The free software movement, started by Richard Stallman with the GNU Project in 1983 and the Free Software Foundation (FSF) in 1985, frames it as an ethical issue and defines four freedoms: to run the program, to study and change it, to redistribute copies, and to distribute modified versions. "Open source" was coined in 1998, when the Open Source Initiative (OSI) was founded, and frames the same idea around practical benefits like quality and collaboration. The OSI's Open Source Definition lists ten criteria a license must meet. People say FOSS or FLOSS (free/libre and open-source software, where "libre" removes the price ambiguity) when they want to include both camps without picking a side. OSS (open-source software) is the shorter, more common term in industry. In everyday developer usage, all four point to the same set of licenses. FOSS licenses fall into two broad families. Permissive licenses (MIT, BSD, Apache 2.0) let you reuse the code in closed-source products as long as you keep the copyright notice. Copyleft licenses (GPL, LGPL, AGPL, MPL) require that distributed changes stay under the same license; the AGPL extends that to software offered over a network. What is not FOSS: "source-available" licenses such as the Business Source License (BSL/BUSL), the Server Side Public License (SSPL) and Elastic License 2.0. You can read the code, but use is restricted, typically for competing hosted services, so neither the OSI nor the FSF accepts them. HashiCorp's 2023 move of Terraform to the BSL is the best-known example; the community fork, OpenTofu, stayed under the open-source MPL 2.0.
When to Use FOSS (Free and Open-Source Software)
Use "FOSS" or "FLOSS" when you mean the whole category and want to stay neutral between the free software and open source camps, for example in policy documents, procurement rules or academic writing. Use "open source" in most developer and business contexts, where it's the expected term. When evaluating a dependency or a self-hosted tool, check the actual license file rather than the word "open" in the marketing. Confirm it appears on the OSI's approved license list (or the FSF's free license list), then check whether it's permissive or copyleft, because that decides what you can do when you ship the code inside your own product. TechLogHub's open-source directory lists each project's license so you can filter before you clone. A quick way to tell the categories apart: - FOSS / open source: source available, and the license allows use, modification and redistribution for any purpose (MIT, Apache 2.0, GPL, AGPL, MPL). - Source-available: source readable, but commercial or competitive use is restricted (BSL, SSPL, Elastic License 2.0). - Freeware: free to download and use, but no source and no right to modify. - Proprietary: no source, paid or free, governed by a vendor EULA. For teams, FOSS matters for three practical reasons: you can audit the code for security issues instead of trusting a vendor, you can self-host and avoid lock-in if a vendor raises prices or shuts down, and you can fix bugs yourself rather than wait for a support ticket. The trade-off is that you own the operations and upgrades unless you buy a hosted or supported edition.
FOSS (Free and Open-Source Software) in Practice
- Linux kernel: the core of most servers and Android, licensed under GPLv2 (copyleft).
- PostgreSQL: a relational database under the permissive PostgreSQL License, similar to MIT/BSD.
- Kubernetes, Apache Kafka and most Apache Software Foundation projects: Apache License 2.0, which adds an explicit patent grant.
- Git, GIMP, Blender and LibreOffice: everyday developer and desktop tools under GPL-family licenses.
- OpenTofu: the MPL 2.0 fork of Terraform created after Terraform's switch to the source-available BSL in 2023.
- Self-hosted open-source alternatives to SaaS, such as Mattermost for Slack, Penpot for Figma and AppFlowy for Notion.
Common Mistakes
- Assuming "free" means free of charge. FOSS is about the freedom to use, change and share; freeware without source is not FOSS.
- Calling source-available projects open source. BSL, SSPL and Elastic License code is readable but restricted, and isn't OSI-approved.
- Treating a public GitHub repo as open source. Code with no license file is all-rights-reserved by default.
- Ignoring copyleft obligations. Shipping modified GPL code in a distributed product means releasing your changes under the GPL; the AGPL also covers network use.
- Assuming the binary you download is FOSS because the project is. Visual Studio Code's source (Code - OSS) is MIT-licensed, but Microsoft's official VS Code builds ship under a proprietary license.