noumenon
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.
The final result of Kant's philosophy, expressed in the concisest terms, was the proposition, so humiliating to human cognition, but, at the same time, so fertile in consequences, that we can know only phenomena, or the outward appearances of things, but not the noumenon, or the thing in itself.
Where it comes from
From German Noumenon, from Ancient Greek νοούμενον (nooúmenon, “thing that is known”), passive present participle of νοέω (noéō, “I know”).
Adapted from the Wiktionary entry noumenon (CC BY-SA 4.0).