As with any other objects that don't really have a single global name used by everyone, different people will call it different things.
Developers who use a particular toolkit might refer to it as a TabPage, ItemContainer or something else (yep - including the CamelCase!).
Designers might use content-pane, content-area, or just simply tab.
The point is that it doesn't matter whether you choose to pinpoint a single phrase, you will always meet people in your business who will call it something else.
Think of it like this: It doesn't matter that I'm a safe driver, there's no accounting for everyone else out on the road.
So whoever you meet, whoever you talk to, in whatever department or role, just make sure that you either agree the terminology that you will use with that person (and this applies to all areas - not just tabs) - or even better: adapt yourself to their world. It's just part of a UX role that you interface with people from many different departments and backgrounds.
If speaking to groups then make your own terminology clear, not in a patronising manner, but just in a casual drop-it-into-the-conversation sort of thing that gets everybody reading off the same (tab) page without requiring a conversation.