reality as it is apart from experience; what remains to be postulated after space, time, and all the categories of the understanding are assigned to consciousness.
British Dictionary definitions for thing-in-itself
thing-in-itself
noun
1.
(in the philosophy of Kant) an element of the noumenal rather than the phenomenal world, of which the senses give no knowledge but whose bare existence can be inferred from the nature of experience
thing-in-itself in Culture
thing-in-itself definition
A notion in the philosophy of Immanuel Kant. A thing-in-itself is an object as it would appear to us if we did not have to approach it under the conditions of space and time.