Something that cannot be known or understood, either because it's impossible to access information about it or because the information doesn't exist.
Compound of 'un-' (not) + 'knowable' (from Old English 'cnawan' to know + '-able' capable of being). The philosophical term became prominent in 19th-century discussions of epistemology and metaphysics.
Philosophers love this word—Immanuel Kant argued that some things (like God, the soul, free will) are fundamentally unknowable because human minds can't access them directly. It's a way of saying 'this is real but forever beyond our grasp.'
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