The Unrepresented
The underlying basis of reality; an "extra-mental" realm, the background of our perceptions, on which our collective representations are based and familiar nature is built. In Barfield's lexicon, comparable to Kant's "noumenal world." In a sense, Barfield hints, the unrepresented is the unconscious about which psychoanalysis speaks.1

"The atoms, protons, and electrons of modern physics," he reminds in Saving the Appearances, "are now perhaps more generally regarded, not as particles, but as notional models or symbols of any unknown supersensible or subsensible base" (17). And in History, Guilt, and Habit, he elaborates on this assumption:

    The physicists, as you know, have been telling us for a long time that, in addition to [the world of common sense], or somehow underlying it, there is the microscopic and submicroscopic world; moreover that it is only this microscopic and submicroscopic world that exists independently of ourselves; only that world that has an objective existence. Some philosophers [G. E. Moore, Bertrand Russell] have added that it is therefore only knowledge of that world that can really be called knowledge at all. (15-16)
Barfield sometimes uses "the unrepresented" interchangeably with inferred nature, "net given," and "the particles."
See in particular Saving the Appearances, passim, Worlds Apart, passim.
1"Certainly, the 'something quite vague' which can be coaxed into producing an atomic explosion does not look much like a collective unconscious--but then neither did the represented, which underlies the ordinary appearances, look like one--until we started thinking seriously about them" (SA 154).