What does 'open source' actually mean?
Open source means software whose source code is available under a licence that lets people read it, change it, use it, and share it with others. The licence is the key part: the code is not merely visible, and it is not ownerless. To count as open source in the formal sense, the licence must fit the Open Source Definition, which the Open Source Initiative (OSI) stewards as the standard reference. Open source is different from freeware, which may cost nothing but remain closed, and from public domain code, where copyright control may no longer apply or has been given up.
What the official definition protects
The formal definition is built around practical freedoms. You can inspect the source code, which means you can see how the software works rather than only running the finished program. You can adapt it, which means you can fix a bug, remove a feature, translate an interface, or make it work in your own environment. You can also redistribute the original or your changed version.
A licence cannot qualify as open source if it gives those rights only to some people or only for some uses. For example, a licence that says “free for education, but not for business” may be generous, but it is not open source in the OSI sense. Open source allows commercial use. Companies can charge for hosting, support, custom work, training, or packaged versions of open-source software. What they cannot do is take away the rights the licence grants.
How the licence does the work
Copyright normally gives the creator control over copying, changing, and distributing software. An open-source licence uses that control to grant permission in advance. That is why the licence matters more than a label on a download page.
Different open-source licences set different conditions. Some are permissive: they let you reuse the code in many settings, including inside proprietary software, as long as you keep required notices. Others are copyleft: they allow reuse and modification, but require certain redistributed versions to remain under the same open terms. Both can be open source if they satisfy the formal definition.
So the question is not only “Can I see the code?” It is also “What does the licence allow me to do with it?” For a developer, a company, a school, or a hobby project, that answer affects sharing, packaging, obligations to credit authors, and whether changes must be released under the same licence.
How it differs from freeware and public domain
Freeware is about price. Open source is about permission. A freeware app may be free to download, but if the source code is hidden and the licence does not allow modification or redistribution, it is not open source.
Public domain is different again. In broad terms, public domain material is not controlled by copyright in the usual way, though the details vary by jurisdiction. That can make it more open in some respects, because people may be able to reuse it without licence conditions. But open source usually keeps copyright in place and uses a licence to grant clear, structured rights.
Why the distinction matters
With proprietary software, the publisher controls the code and usually keeps it private. You may be allowed to run the program, but not to inspect how it works, repair it yourself, or share modified versions. Open source changes that relationship. It gives users and developers a legal path to examine, adapt, and redistribute the software.
That does not mean every open-source project is safe, well maintained, or right for every use. It means the rules are visible and the code can be checked. The practical next step is to read the project’s licence, because “open source” tells you the door is open, while the licence tells you exactly how you may walk through it.