n. One who believes in nothing; one who advocates the metaphysical doctrine of nihilism.n. One who rejects all the positive beliefs upon which existing society and governments are founded; one who demands the abolition of the existing social and political order of things.n. Specifically An adherent of nihilism; a member of a Russian secret society which aims at the overthrow of the existing order of things, social, political, and religious; a Russian anarchist or revolutionary reformer. See nihilism, 4.