https://opensource.stackexchange.com/questions/8367/is-the-term-open-source-a-trademark has a discussion about this.

The short story is that the OSI failed to obtain a legal trademark in the US for the term "open source" (software), resulting in many opportunistic companies and individuals adopting the term popularized by the OSI (which was founded by Eric Raymond, Michael Tiemann and Bruce Perens).

There was controversy at the time due to it being a business-friendly spin on the ideological "free software", and I personally avoided using the term for many years as a result. Even without a trademark on the now generic term of Open Source, there is still value in the OSI brand and its stamp of approval on a license.

Those who want to be crystal clear, should probably always say OSI Approved Open Source License.

Now, I'm off to have a Nescafé Approved Coffee.