This is the meaning for which I've taken it as my user name - something missing, a gap. It can be used more in the philosophical sense - which is how I view life; with 'something' missing that everyone is looking for but no one can find.
This is not how I meant it - but it doesn't mean you're not right. I haven't done biology for about six years - so it's a little difficult to say either way. Blastula, osmosis, reticular activating system, mitochondria, adenosine triphosphate - that's about all I can remember (good effort after three years of studying huh? Money well spent obviously)