1. in common usage, a state of opposition or contradiction between two things.

2. in the thought of Immanuel Kant, a contradiction between two a priori propositions in metaphysics, each of which can be supported by equally valid proofs. Kant cited a series of such antinomies as part of his argument against speculative metaphysics and in favor of his own position that the only world one can have knowledge of is the world of phenomena.