I can't tell whether this is serious or sarcastic. It lacks the "/s" at the end, so I'll assume that you're serious.
> but it's not open source unless you know the OSI's definition.
> Open source is an agreed upon definition for software provided under an OSI or FSF approved license.
Open source software (by today's standards) existed before the OSI or even the term "Open Source" existed - the creators of BSD *nix, X11 and TeX all chose a liberal, open source license before a cabal around Eric S. Raymond would decide on a definition of Open Source and the term itself. The idea came after the actual thing existed, and the definition after the idea. So it would be weird to call something Open Source that doesn't fit the Open Source Definition (because the latter was conceived together with the term), but it would absolutely be possible for a software/license to be Open Source without knowing about the Open Source Definition, as is the case with BSD, X11, TeX and many Free Software programs.
Next, there is the idea of the Open Source Initiative approving Open Source licenses, which is a good thing because people can trust any OSI-approved license to be an Open Source license. However, not every Open Source license is OSI-approved, because OSI applies additional criteria (e.g. being reusable).
So, yes, it's possible for a license to be Open Source but not OSI approved, when it fits the Open Source Definition but hasn't been submitted, not reviewed, or doesn't meet the additional criteria set forth by the OSI.
The fact that your employer's legal department will mistrust your personal judgement of software fitting the Open Source Definition (for perfectly sensible reasons) doesn't make software not Open Source (any more than a US State can decide to make Pi a rational number or to make dolphins be fish).