n. In metaphysics, the doctrine that nothing can really be known, because nothing exists; the denial of all real existence, and consequently of all knowledge of existences or real things.n. In theology, same as nihilianism.n. Total disbelief in religion, morality, law, and order.n. Originally, a social (not a political) movement in Russia, in opposition to the customary forms of matrimony, the parental authority, and the tyranny of custom. In this sense the word was introduced by Turgeneff in 1862. See nihilist, 3.n. Later, a more or less organized secret effort on the part of a large body of malcontents to overturn the established order of things, both social and political.