I see this term used in ways that make me think that its definition is similiar to "awareness" the way the tibetans use it, or "true nature" or "Mind" the way the zennies use it, or "emptiness" the way it used in mahayana, that is as a positive something that might be better called everythingness.
Then again, i see most of the concepts above refuted and picked apart. So what do theravadans mean when they refer to "the deathless".
“The truth knocks on the door and you say, "Go away, I'm looking for the truth," and so it goes away. Puzzling.” ― Robert M. Pirsig