I don't want to " Thread Jack " ANY threads, so I am wondering what a definition would be?

I believe going off topic with a totally different question // topic would fall in that definition.
If a Q is related but not exactly on par with the thread, is that falling under the definition?

 

Have you received a Moderation notice about thread Hi-Jacking to prompt your question?

As far as the question itself, taking an existing thread and changing the topic/direction of the thread would be a definition of thread hi-jacking. It probably becomes an issue if it doesn't help the OP (original poster) who created the thread that still needs help.

Each situation can be different when you involve all types of different people. But if you hi-jack a thread and take away help from the OP, then it's most likely to be an issue to the OP.