[thomascountz]

They own the trademark "Open Souce Initiative," which they say you can use with no advanced written permission if you follow all specific guidelines, including:

> the use of the term “Open Source” is used solely in reference to software distributed under OSI Approved Licenses. [1]

So you can refer to any software as "Open Source," regardless of their definition. But, if you call a piece of software "Open Source" alongside the use of the Open Source Initiative's trademark, then you must also use their definition of "Open Source," unless you otherwise have written permission.

[1]: https://opensource.org/trademark-guidelines