noumenon

In the philosophy of Kant, an object as it is in itself independent of the mind, as opposed to a phenomenon.

In the Kantian philosophy:

That which can be the object only of a purely intellectual intuition.

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.

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.

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 intellectual conception of a thing as it is in itself, not as it is known through perception