AHRIMAN ä’ rĭ mən. The Middle Pers. form of the title of the evil principle in the cosmos according to the Avesta, the book of Zoroastrianism (Mazdaism), in which he is known as Angra Mainyav, “Evil Spirit.” He is the equal and opposite of the “Good Spirit,” Ahura Mazdā, in the great system of dialectic dualism.