Except.. it kinda is. I mean yes, it is obviously true that words can (and do) have multiple meanings. But within a given context, there are "generally accepted" definitions and there are idiosyncratic / obscure / unusual definitions. And in the kind of context we're talking about here, to use "Open Source' to mean anything other than what is defined by the OSD is to intentionally choose a non-standard, idiosyncratic definition. It wouldn't be reasonable for one to do that and then expect everybody else to understand / agree with their non-standard usage.