OK, here we are:

Chapter XXXIII 

(33 or page 370 in my paperback)

Hilda speaking to Zeb:

"'Don't laugh, because you told me that you and Jacob discussed the heart of it, the idea that human thought exists as quanta.  I don't know quanta from Qantas Airways, but I know that a quantum is an indivisible unit. You told me that you and Jacob had discussed the possibility that imagination had its own sort of indivisible units or quanta - you called them "fictons" - or was it ficta?  Either way, the notion was that every story ever told - or to be told if there is a difference - exists somewhere in the Number of the Beast."

So, a basic unit of imagination.  Hokum of course, but I hope it makes it into Jeff's NotB rules.  :-)

tc