Literally, "The Personally received teaching".
That is, a teaching not derived from books or doctrine, or even oral traditions, but one directly experienced.
So this then, is the science of exploring the inner world. We all have our (subjective) interpretation of reality.
It, as a coined term, and an "organized" science, dates back to the 1st century or so, though, perhaps much earlier. It possibly has Egyptian influences, and almost certianly Greek touches.
A primary focus of any "study" of Kaballah is
Alternative spellings: Cabala, Qaballah.
Check out William Gray's book "The Ladder of Lights" for a good introduction.