The Collaborative International Dictionary of English v.0.48:

Noumenon \Nou"me*non\ (nou"m[-e]*n[o^]n), 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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

noumenon
    n 1: the intellectual conception of a thing as it is in itself,
         not as it is known through perception [syn: noumenon,
         thing-in-itself]

The Devil's Dictionary (1881-1906):

NOUMENON, n.  That which exists, as distinguished from that which
merely seems to exist, the latter being a phenomenon.  The noumenon is
a bit difficult to locate; it can be apprehended only be a process of
reasoning -- which is a phenomenon.  Nevertheless, the discovery and
exposition of noumena offer a rich field for what Lewes calls "the
endless variety and excitement of philosophic thought."  Hurrah
(therefore) for the noumenon!