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.
2009 Update: The actual object denoted by a representational noun- in this case, a noun.