Nihilistic. ni·hil·ism ( P ) Pronunciation Key (n-lzm, n-) Philosophy. An extreme form of skepticism that denies all existence. A doctrine holding that all values are baseless and that nothing can be known or communicated. Rejection of all distinctions in moral or religious value and a willingness to repudiate all previous theories of morality or religious belief. The belief that destruction of existing political or social institutions is necessary for future improvement. also Nihilism A diffuse, revolutionary movement of mid 19th-century Russia that scorned authority and tradition and believed in reason, materialism, and radical change in society and government through terrorism and assassination. Psychiatry. A delusion, experienced in some mental disorders, that the world or one's mind, body, or self does not exist.