1) total rejection of established laws and institutions.
2) anarchy, terrorism, or other revolutionary activity.
3) total and absolute destructiveness, esp. toward the world at large and including oneself: the power-mad nihilism that marked Hitler’s last years.
4) Philosophy:
a. an extreme form of skepticism: the denial of all real existence or the possibility of an objective basis for truth.
b. nothingness or nonexistence.
5) sometimes initial capital letter: the principles of a Russian revolutionary group, active in the latter half of the 19th century, holding that existing social and political institutions must be destroyed in order to clear the way for a new state of society and employing extreme measures, including terrorism and assassination. 6) annihilation of the self, or the individual consciousness, esp. as an aspect of mystical experience.