n. In the philosophy of Immanuel Kant (1724-1804) and those whom he influenced, a thing as it is independent of any conceptualization or perception by the human mind; a thing-in-itself, postulated by practical reason but existing in a condition which is in principle unknowable and unexperienceable.
the GNU version of the Collaborative International Dictionary of English
n. 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.
The Century Dictionary and Cyclopedia
n. In the Kantian philosophy:
n. That which can be the object only of a purely intellectual intuition.
n. Inexactly, a thing as it is apart from all thought; what remains of the object of thought after space, time, and all the categories of the understanding are abstracted from it; a thing in itself.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. the intellectual conception of a thing as it is in itself, not as it is known through perception
Word Usage
"If, by the term noumenon, we understand a thing so far as it is not an object of our sensuous intuition, thus making abstraction of our mode of intuiting it, this is a noumenon in the negative sense of the word."