(Sanskrit अहिंसा) Literally, "not harming," usually interpreted as nonviolence.
"Ahimsa, nonviolence, is the pure thought of India; ahimsa is indeed inspired by universal love. The word himsa means “to want to kill; to want to harm.” Therefore, ahimsa is the renunciation of all death or harm caused by violence or even such intentions. Ahimsa is the opposite of egotism; it is absolute altruism and love; it is upright action... Ahimsa is nonviolence in thought, word and deed. Ahimsa is respect for others’ ideas, respect for all religions, schools, sects, organizations, etc." - Samael Aun Weor, Revolution of the Dialectic
Ahimsa is the first restraint (yama) of raja yoga.
"Patanjali's meaning [of Ahimsa] was the removal of DESIRE to kill. This world is inconveniently arranged for a literal practice of AHIMSA." - Sri Yukteswar