Open Source

Open source software is software whose source code is made publicly available for anyone to view, use, modify, and distribute under a license that complies with the Open Source Definition (OSD), established by the Open Source Initiative (OSI).

Category: General (also: Software Engineering, Legal)

Author: Atomic Glue Technical Team

## Definition

Open source refers to software distributed with a license that grants anyone the right to use, study, modify, and share the source code. The Open Source Initiative (OSI) defines open source through the Open Source Definition (OSD), a set of ten criteria that a license must meet to be certified as open source. These include free redistribution, access to source code, allowance of derived works, and no discrimination against fields of endeavor. Open source is not the same as "free software" (though they overlap significantly); free software emphasizes ethical freedoms, while open source focuses on practical benefits of collaborative development. Major open source licenses include MIT, Apache 2.0, GPLv3, and BSD. The open source model powers the majority of modern software infrastructure, from Linux and Kubernetes to Python and WordPress.

## The Open Source Definition (OSD)

The Open Source Initiative's ten criteria for open source licenses: 1. **Free Redistribution:** The license must not restrict anyone from selling or giving away the software. 2. **Source Code:** The program must include source code and allow distribution in compiled and source form. 3. **Derived Works:** The license must allow modifications and derived works. 4. **Integrity of Author's Source Code:** The license may require that modifications are distributed as patches. 5. **No Discrimination Against Persons or Groups:** The license must not discriminate against any person or group. 6. **No Discrimination Against Fields of Endeavor:** The license must not restrict usage in a specific field (e.g., business, research). 7. **Distribution of License:** The rights attached to the program apply to all who receive it. 8. **License Must Not Be Specific to a Product:** The rights must not depend on the program being part of a particular distribution. 9. **License Must Not Restrict Other Software:** The license must not restrict other software distributed alongside it. 10. **License Must Be Technology-Neutral:** No provision may be based on individual technology or interface style.

## Open Source vs Proprietary Software

The fundamental difference is control. Open source gives users control over the software they run. If a feature is missing or a bug exists, users can fix it themselves or hire someone to fix it. Proprietary software keeps the source code secret; users are dependent on the vendor for updates and fixes. Open source also tends to have lower upfront costs, massive community support ecosystems, and higher transparency (anyone can audit the code for security issues). Proprietary software often provides dedicated support, polished user experiences, and liability protection through commercial agreements. The choice depends on your organization's resources, risk tolerance, and need for customization.

## Common Open Source Licenses

Licenses fall on a spectrum from permissive to copyleft: * **MIT License:** Extremely permissive. Users can do almost anything with the code, including using it in proprietary software. Simple and widely adopted. * **Apache 2.0:** Similar to MIT but includes an express grant of patent rights and requires modified files to carry notices. * **BSD 2/3-Clause:** Permissive, similar to MIT. The 3-clause version includes a non-endorsement clause. * **GPLv3 (and GPLv2):** Copyleft. Derivative works must also be distributed under GPL. This ensures modifications remain open. * **LGPL:** A weaker copyleft that allows linking from proprietary software. * **AGPL:** GPL with a network clause. Code accessed over a network counts as distribution, closing the "application service provider loophole."

## Common questions

Q: Is open source software free?

A: Open source is free as in freedom (not necessarily price). Most open source software is available at no cost, but businesses can charge for distribution, support, and consulting.

Q: Can I use open source code in commercial products?

A: It depends on the license. Permissive licenses (MIT, Apache, BSD) allow it. Copyleft licenses (GPL) require that derivative works also be open source.

Q: What is the difference between open source and free software?

A: Free software emphasizes ethical freedom (the user's right to control the software). Open source emphasizes practical development benefits. The definitions overlap heavily, but the philosophies differ.

## Key takeaways

• Open source software is publicly available for use, modification, and distribution under OSI-approved licenses.
• Licenses range from permissive (MIT, Apache) to copyleft (GPL, AGPL), each with different requirements.
• Open source powers the majority of modern software infrastructure.
• Choose a license based on whether you want to allow proprietary use or require modifications to stay open.

## Related entries

• [SaaS / IaaS / PaaS](atomicglue.co/glossary/saas)
• [Semantic Versioning (SemVer)](atomicglue.co/glossary/semantic-versioning)
• [Markdown](atomicglue.co/glossary/markdown)

Last updated July 2025. Permalink: atomicglue.co/glossary/open-source