These are the guidelines for using "OSI", "Open Source Initiative", and the OSI logo. In other words, this is the OSI's definition of their version of open source, but the term "open source" has been around for much longer than OSI.

How much longer?

“The first example of free and open-source software is believed to be the A-2 system, developed at the UNIVAC division of Remington Rand in 1953,[6] which was released to customers with its source code.”

https://en.m.wikipedia.org/wiki/History_of_free_and_open-sou....

That doesn't suggest they used the term "open source".

Looks like not much: https://opensource.com/article/18/2/coining-term-open-source...

That's the FAQ relating to the use of the OSI trademark.

You might be better off linking to the definition of open source, per the OSI, since 2006:

https://opensource.org/osd

Eitherway, this is why open source zealots tend to specifically say F/LOSS (free / libre open source software), to avoid any ambiguity with people who like to claim 'open source' is ambiguous and easily conflated with 'source available'.

...That there are so many of them proves their own point, I guess!