In simple terms
Open-source software is software whose source code, the human-readable instructions behind the program, is published under a license that lets anyone view, modify, and redistribute it. [1] That stands in contrast to proprietary software, where the vendor keeps the source code private and only distributes the compiled program.
How it works
The Open Source Initiative maintains the Open Source Definition, the standard most open-source licenses are checked against. It requires that the software can be freely redistributed, that source code is available, and that anyone may create and distribute modified versions. [1] Beyond that baseline, licenses differ. Permissive licenses such as MIT or Apache place few restrictions on reuse, including in proprietary products. Copyleft licenses such as the GNU General Public License go further, requiring that any distributed derivative work stay open under the same terms. [2]
Most large open-source projects are built through public collaboration: volunteers and paid contributors from many organizations propose changes, which maintainers review before merging them into the shared codebase. [2]
Why it matters
Open-source code can be inspected and audited by anyone, which supporters argue makes security flaws easier to find than in closed systems where outsiders cannot look at the code at all. [2] It also lets organizations avoid being locked into one vendor’s software and pricing, and lets smaller developers build on infrastructure they could not have built themselves.
Where you’ll see it
- The Linux operating system kernel, which runs most of the world’s servers and supercomputers [2]
- Web browsers and web servers built on open-source foundations
- Publishing platforms such as WordPress, which itself runs on open-source code [2]
- Programming languages and the libraries developers use to build other software
Example
A company can take the open-source Linux kernel, adapt it for its own product, and ship that product, provided it follows the terms of the kernel’s license.
Often confused with
Free software and open-source software overlap heavily but come from different traditions: the free software movement frames access to source code as an ethical right, while the open-source label was coined to emphasize practical development benefits. In practice, most licenses satisfy both definitions.
Key facts
- The Open Source Initiative maintains the Open Source Definition, the widely used standard that open-source licenses are checked against.1
- The definition requires free redistribution, access to source code, and the right to make and distribute modified versions.1
- Well-known open-source projects include the Linux kernel, the Firefox browser, and the WordPress publishing platform.2
- Open-source licenses vary in how much they restrict reuse, from permissive licenses like MIT to copyleft licenses like the GNU General Public License, which requires derivative works to stay open as well.2
Quick checkWhat does an open-source license actually guarantee, beyond letting you see the code?Show answer
The right to freely redistribute the software and to modify it and distribute your own modified version, not just the ability to view the source.
Sources
- Open Source Initiative. The Open Source Definition. Undated (accessed 1 November 2026)
- IBM. What Is Open Source?. Undated (accessed 1 November 2026)
Editorially reviewed by Last reviewed November 1, 2026Researched and drafted with AI assistanceReport an issue