Is Open Source Software a subset of free software?
I was recently asked this and realized I was unsure the exact answer. I understand that free software is not a type of open source software, but I am unsure if the reverse is true or not.
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"Free software" and "open source software" are two terms for the same thing: software released under licenses that guarantee a certain, specific set of freedoms.
Open Source arose from people who supported the FSF, but later branched out over the decision of philosophy and marketing. There are differences, of course, such as the FSF has a 4-point definition for software freedom, while the OSI has 10. The FSF is sort of like a parent to the OSI.
Generally, we refer to software that is with Free Software and Open Source to be FOSS (Free and Open Source Software), or FLOSS (Free/Libre/Open Source Software)
Pandya raised this in chat: Looks like something where the FSF doesn't agree with Open Source.
It depends on the definitions of the terms "Open Source Software" and "Free Software".
A common definition (and in my opinion it’s the one that should be used, otherwise the scope of the terms will be subjective):
Free Software is software licensed under an FSF-approved software license.
(on the basis of The Free Software Definition)
Open Source Software is software licensed under an OSI-approved software license.
(on the basis of The Open Source Definition)
In most cases, a Free Software license is also an Open Source Software license, and vice versa. But there are exceptions.
For a definite and up-to-date answer, you should use the original sources and compare if both organizations, the FSF and the OSI, have approved each license:
In the Wikipedia article "Comparison of free and open-source software licenses", there is an "Approvals" table where you can see this at a glance (but this information might not be accurate).
↳ So these early versions of Mozilla are Free Software, but not Open Source Software.
↳ So Active Agenda is Open Source Software, but not Free Software.
The main difference is philosophical, and so subtle that it is easy to miss because it is not in any licence or list. It is that open source tends emphasize quality, reviewability, correctness and community whereas free software focuses on personal liberty, speech, and opportunity. Both camps endorse the others goals as good, just not quite as important to software and share methodologies and the belief that the ability to see, modify, and distribute source code is critical to their goals. Many projects have people from both camps as well as people who just contribute code and leave the licences to someone who cares.
Given that the OpenSource Definition lists 10 points and the "free" software definition lists only four is a hint that free software is a smaller subset of OpenSource and may violate the OSS rules.
Given that the OSI in former times listed the GPL as a non-OpenSource compliant license before the FSF assured that the ambiguous parts of the GPL have to interpreted in a way that is compliant to the OSS definition creates the impression that the free software definition is a weaker definition.
Different organisations attempt to determine the terms "open source" and "free software. The defintions are often similar in basic spirit but differ in the details and interpretations and are not a strict superset or subset of each other.
And none of these orgnaisation have a monopoly over the english language. So individuals may use the terms in ways that differ from what any of the well known organsiations say. Particulally those outside the "free software community" may use "free software" to reffer to software that is free of charge but may be heavilly incumbered.