Well, specifically, it implies an open source license under the OSI definition, which happens to be almost identical in a licenses covered to the Free Software definition from the FSF.

It might to you, but it shouldn't. The definition of open source is not controlled by the OSI; they only organize licenses and certify licenses as OSI-approved. However, you don't need an OSI-approved license to be open source. There are many open source packages that are not under an OSI license--most famously, SQLite.

*edited for clarity and a typo.

You don't need to use an OSI-approved license to be Open Source, but you need to use a license that complies with OSI's Open Source definition. Otherwise, the term would be meaningless.

Well, since they invented the term, to a certain degree they do.

They didn't invent the term. The term was invented first. The OSI was founded later. See the OSI's own post on this [1].

A much more detailed discussion of the origin of the term and its initial use appears here [2]. The latter link in particular is interesting reading, because it includes the political dimensions (especiall w.r.t O'Reilly's difficulties with the FSF).

[1] https://opensource.org/history [2] http://www.oreilly.com/openbook/freedom/ch11.html

https://news.ycombinator.com/item?id=13971909 :)