Is "Open Source" Hyphenated?

Well, from the title you should get a pretty good idea of the answer if you believe in Betteridge’s Law! No, open source is not hyphenated when it’s referring to the software domain. If it were purely a descriptive adjectival phrase adding attributes to the noun “software” then it would be hyphenated, to clarify that the parts of the adjectival phrase do not modify each other but rather modify the target noun. But instead, it is a noun phrase, or even part of the noun phrase “open source software” with the “software” dropped for concision. Those don’t have hyphens because there is nothing to clarify by doing so. It was intentionally coined as a noun phrase.

This noun phrase represents the generally-accepted term of art where software is pre-licensed so that it comes with all rights necessary to permit enjoying open source software in any way and for any purpose – including using, studying, improving, sharing and monetising – without negotiating with its creators.

By contrast, the term open-source is generally used to describe military and diplomatic intelligence that is obtainable from open as opposed to classified sources such that they are readable without privilege, such as by the press or the open Internet. This usage has been common for many decades and continues in parallel with the noun-phrase usage associated with software. This usage is definitely an adjectival phrase so definitely has a hyphen.

So when you see open source it will be about open source software – with the freedoms left attached – whereas when you see open-source it will be about open-source intelligence gained from open sources.