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 by a process of reasoning -- which is a phenomenon.
Source: The Devil's Dictionary, 1911.
-- Ambrose Bierce, (Jun 24 1842-1914), US journalist, short-story writer; He was a noted wit, satirist and sardonic short story writer who disappeared while covering the Mexican revolution.