“Open Source” is the proper name of a campaign to promote the pre-existing concept of Free Software to business, and to certify licenses to a rule set. Please treat it as a proper name, rather than a descriptive term, when it is applied to any work to which a license compliant with the Open Source Definition could possibly apply. That means you should capitalize both words.
“Open Source software” is a subset of the general category of software, and can be written without capitalizing the word “software” where appropriate.
For a work to be Open Source, it must be entirely under a license or licenses which comply with the Open Source Definition.
If someone says that something is Open Source and the license does not comply with the Open Source Definition, please tell them that the proper definition of Open Source is a work under a license which complies with the Open Source Definition. Be firm but polite.
When “Open Source” is used as a descriptive term rather than a proper name, it becomes a fuzzy reference to a development paradigm with no concrete definition, rather than the specific set of license rules in the Open Source Definition. So, it can be made to mean just about anything. Don’t allow people to erode the definition of Open Source.
Where Did Open Source Come From?
Obviously, Richard Stallman was first with Free Software, and we stand on Richard’s shoulders.
The text of the Open Source Definition is taken directly from the Debian Free Software Guidelines (DFSG). I created the DFSG and the Debian Social Contract, and submitted the first draft to the Debian developers on their mailing list in June of 1997. I edited it, taking suggestions and building consensus on that same mailing list, for about a month and then announced the result as project policy.
The inspiration for the Debian Social Contract and DFSG was two events. The first was questions on the Debian developer mailing list regarding which licenses were acceptable into Debian (Erik Andersen says he asked the question which inspired me). The second was an email conversation between Ean Schuessler and Donnie Barnes (one of the early Red Hat employees). In his email to Donnie, Ean accused Red Hat of never having elucidated their social contract with the Free Software community. Ean did not actually suggest that Debian form a social contract, he only complained about Red Hat’s lack of one, perhaps believing that Debian’s social contract was clear. I decided to create one.
Eric Raymond approached me with the intention of forming the Open Source Initiative, I believe on February 2, 1998. Eric had been visiting California to see Larry Augustin of VA Linux Systems, and a meeting was held at VA where Christine Peterson (of the Foresight Institute and then married to Eric Drexler) suggested “Open Source” as a way to promote Free Software without the stigma of “free” in the English language. Some references date the VA meeting as February 3, I suspect that’s incorrect.
I suggested to Eric that I use the already-written Debian Free Software Guidelines as the Open Source Definition. Eric agreed. That’s the only input he had, as the text had already been written and adopted as the DFSG eight months previously and was not modified other than removal of Debian references. I wrote an announcement of Open Source which was published on February 9, and that’s when the world first heard about Open Source.
By the way, the 20th anniversary of Open Source is February 9, 2018.
I’ve been asked whether I used the Four Freedoms of the Free Software Foundation when writing the DFSG. I didn’t. First, the Four Freedoms didn’t exist then, they were the Three Freedoms! These had been published in the first “GNUS Bulletin” by Richard Stallman, which was distributed in paper form around MIT. I had read Richard’s announcement of Free Software on a USENET newsgroup but might not have read the Three Freedoms at that time. At the time I wrote the DFSG, the web was in its infancy and FSF had built a web site just months before. I simply didn’t have a copy of the Three Freedoms in hand. The DFSG was mainly inspired by the licenses that Debian was already using, and of course the GPL, created by Richard, was among them.
I wrote to Richard with a new copy of the DFSG and he replied “this is a good definition of Free Software.” He didn’t suggest that I use the Three Freedoms.
It is unfortunate that for some time the Open Source Initiative deprecated Richard Stallman and Free Software, and that some people still consider Open Source and Free Software to be different things today. I never meant it to be that way. Open Source was meant to be a way of promoting the concept of Free Software to business people, who I have always hoped would thus come to appreciate Richard and his Free Software campaign. And many have. Open Source licenses and Free Software licenses are effectively the same thing.