As an example, the real time operating system FreeRTOS is licensed under the FreeRTOS Open Source License which is based on a modified GNU GPL, the modification taking the form of an exception. The GNU GPL is approved by the Open Source Initiative, but not the FreeRTOS Open Source License. If I use FreeRTOS in my software, and the rest of my software is released under an approved Open Source licence, would I be wrong to call my software Open Source?
The OSI FAQ says:
Can I call my program "Open Source" even if I don't use an approved license? Please don't do that. If you call it "Open Source" without using an approved license, you will confuse people. This is not merely a theoretical concern — we have seen this confusion happen in the past, and it's part of the reason we have a formal license approval process. See also our page on license proliferation for why this is a problem.
The Open Source stamp is important to me and I like using FreeRTOS but I also want to call things what they are. I am not 100% sure about this one that is why I prefer to ask here. I asked the question in a generic way so it can also be useful to others.