noun

• (usually uncountable) The view that all endeavours are devoid of objective meaning.
• (usually uncountable) The rejection of, or opposition to, religious beliefs, (inherent or objective) moral principles, legal rules, etc., often due to the view that life is meaningless (sense 1).
• (usually uncountable, politics) The rejection of non-proven or non-rationalized assertions in the social and political spheres of society.
• (uncountable, psychiatry) A delusion that oneself or the world, or parts thereof, have ceased to exist.
• (countable, uncountable, philosophy) A doctrine grounded on the negation of one or more meaningful aspects of life; in particular, the view that nothing in the world actually exists.
• (countable) Something that is regarded as meaningless.
• (Russia, politics, historical) A movement of the 1860s that rejected all authority and promoted the use of violence for political change.
• (uncountable, Russia, politics, historical) Alternative letter-case form of Nihilism (“a Russian movement of the 1860s that rejected all authority and promoted the use of violence for political change”) [(Russia, politics, historical) A movement of the 1860s that rejected all authority and promoted the use of violence for political change.]