Zo·ro·as·tri·an·ism (zôr??-?s?tr?-?-n?z??m)n. The religious system founded by Zoroaster and set forth in the Avesta, teaching the worship of Ahura Mazda in the context of a universal struggle between the forces of light and of darkness.Zo?ro·as?tri·an adj. & n.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.Zoroastrian (?z?r???æstr??n) adj (Other Non-Christian Religions) of or relating to Zoroastrianism or Zoroastern (Other Non-Christian Religions) a follower of Zoroaster or adherent of Zoroastrianism: in modern times a Gabar or a ParseeCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Zo?ro?as?tri?an (?z?r o??æs tri ?n, ?zo?r-) adj. 1. of or pertaining to Zoroaster or to Zoroastrianism. n. 2. an adherent of Zoroastrianism. [1735?45;