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I think that when a software is proprietary, then its source code is "legally defended" and hence not released to the general public in any way.

Are the terms proprietary software and closed source software synonymous?

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No, “proprietary software” is not necessarily “closed source” – there is also software that is proprietary yet still has source code publicly available. But it depends on exact definitions.

Open Source software describes programs that provide their recipients with Software Freedom. This includes the right to use the software for any purpose, to modify it, and to share it in original or modified form under the same terms. Access to the source code is precondition for being able to modify the software and thus a precondition for Open Source, but access to source code is not the defining feature of Open Source.

Open Source licenses work because the software is copyrighted. It is not public domain. However, public domain software can also be Open Source if the source code is available.

Proprietary software is software that is not Open Source. That means it is not available under licenses that provide Software Freedom.

Proprietary vs Open Source is not a completely binary choice, it is more of a spectrum. Whereas Open Source is clearly defined, there are many variants of proprietary software with some degree of openness.

  • This includes software that is truly closed source because the software is never externally distributed (e.g. backoffice or SaaS software), or software binaries that are made available to users under a non-Open license.
  • For some software, source code is made available subject to NDAs or other contracts, e.g. so that partners can debug their software. The original UNIX operating system was always distributed in source form. Microsoft makes Windows source code accessible to some researchers.
  • But there are also cases where proprietary source code is made publicly available. This might be done for a community edition of the software, or in order to enable public review of the software.

Proprietary software where the source code is publicly available is sometimes called Shared Source or Source Available. Such software is still proprietary and cannot be used or modified for any purpose. In particular, software without any license can only be looked at, but not modified or copied: the default license under copyright is “all rights reserved”.

There are variants of software that are neither Open Source nor quite proprietary. The term “proprietary” implies that the rights are retained by the software's owner. However, there is software without such a singular owner (so not proprietary), but also without providing Software Freedom to recipients (so not Open Source). Such licenses are also covered the Source Available term, though there is a need for a separate term to accurately describe this space. Here, we often see licenses that are mostly open but want to prevent certain uses, e.g. the SSPL license, the Creative Commons NonCommercial license variants, or licenses from the Ethical Source movement.

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  • Would you say that proprietary software is not necessarily closed source software but that closed source is necessarily proprietary software? – lahmedi Feb 28 at 9:45
  • @lahmedi yes! The point of this answer is that “proprietary” also contains source available software, so the term “closed source” could be misleading. – amon Feb 28 at 11:02
  • I thank you dearly, amon, for replying back; I have suggested this edit and I kindly ask from you to review it in your free time (link in case a notification was deleted for you): opensource.stackexchange.com/review/suggested-edits/7731 – lahmedi Feb 28 at 13:59
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    Even a fully closed-source application can have multiple copyright holders. For example, when vendor A provides a closed-source library that vendor B uses in their application. – Bart van Ingen Schenau Mar 2 at 7:20

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