Source : Webster's Revised Unabridged Dictionary (1913)

Noumenon \Nou"me*non\, n. [NL. fr. Gr. ? the thing perceived, p.
   pr. pass. of ? to perceive, ? the mind.] (Metaph.)
   The of itself unknown and unknowable rational object, or
   thing in itself, which is distinguished from the {phenomenon}
   through which it is apprehended by the senses, and by which
   it is interpreted and understood; -- so used in the
   philosophy of Kant and his followers.

Source : WordNet®

noumenon
     n : the intellectual conception of a thing as it is in itself,
         not as it is known through perception [syn: {thing-in-itself}]
     [also: {noumena} (pl)]