The Collaborative International Dictionary of English v.0.48:

Ahriman \Ah"ri*man\, n. [Per.]
   The Evil Principle or Being of the ancient Persians; the
   Prince of Darkness as opposer to Ormuzd, the King of Light.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Ahriman
    n 1: the spirit of evil in Zoroastrianism; arch rival of Ormazd