The term “Open Source” has a precise definition, determined by the Open Source Initiative, who invented the term and started promoting it. Nobody used this term in software contexts before they did. They get to define it.
Not sure why this is downvoted. One of the criteria when the term "open source" was decided on by the people who went to form the OSI was that it must not have (much) documented earlier use in order to become eligible for trademark protection.
It was always meant to be a trademark, even if they ended up losing that particular battle. It was deemed to descriptive and generic to be trademarkable, IIRC. The OSI published their criteria anyway, enforced it as much as they could, and really publicized the term over the years. Of course they get to define it.