Thing-in-itself

Kant's name (das Ding an sich) for reality as it is independent of how we experience it. Per the English Wikipedia article on critique-of-pure-reason (T2), such things cannot be known: experience is always structured by the forms of intuition, space and time, and by the concepts of the understanding. What we know are appearances, which are representations.

Transcendental idealism and empirical realism

The thesis that space and time belong to our intuition and not to things in themselves is Kant's transcendental idealism. Wikipedia separates it from Berkeley's idealism by pairing it with empirical realism: appearances are real as objects of experience. This wiki does not give transcendental idealism its own page; the Copernican-turn reading on the main page and this one cover it.

The standing problem

Wikipedia flags an apparent paradox: Kant still treats the thing-in-itself as the cause of what appears to us, yet causality is one of the categories-of-understanding, which he limits to experience. immanuel-kant already records the neighbouring dispute from sep-kant: whether appearances and things in themselves are two classes of object or one class seen under two aspects is still argued. This page does not take a side.

The distinction also does work elsewhere in the book: it is what lets both sides of the third and fourth antinomies-of-pure-reason stand.

Related

critique-of-pure-reason · immanuel-kant · sep-kant · antinomies-of-pure-reason