Wiktionary
n. (context philosophy English) In the philosophy of http://en.wikipedia.org/wiki/Immanuel_Kant (1724-1804) and those whom he influenced, a thing as it is independent of any conceptualization or perception by the human mind, postulated by practical reason but existing in a condition which is in principle unknowable and unexperienceable.
WordNet
n. the intellectual conception of a thing as it is in itself, not as it is known through perception [syn: noumenon]
Usage examples of "thing-in-itself".
The same disconnectedness it discovered between the Thing-in-itself and the rest of the world, applied equally to the moment by moment perception of the Thing-in-itself.
In its place he substituted the power of the productive imagination of the infinite and supra-individual Self, which in essence meant: it is not that there is some forever unknowable thing-in-itself, quite different from consciousness, that impinges on consciousness and "causes" perception.
To return to the First Noble Truth, both physical and mental suffering are to be recognized, but according the Madhyamaka view, neither exists as a thing-in-itself, and therefore the dualism between them is of a relative, not an absolute, nature.
It was a recurrent feeling with me, one that I would always forget until I again realized the same thing: the puniness of humanity against the immensity of that thing-in-itself which I had seen reflected in the mirror.