Zo·ro·as·ter (zôr??-?s?t?r) or Zar·a·thu·stra (z?r??-tho?o?str?) In Zoroastrian tradition, the Iranian prophet who founded Zoroastrianism. The Avesta describes his reception of a revelation from Ahura Mazda and other events in his life, traditionally dated to the 6th century bc.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.Zoroaster (?z?r???æst?) n (Biography) ?628??551 bc, Persian prophet; founder of Zoroastrianism. Avestan name: Zarathustra Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Zo?ro?as?ter (?z?r o??æs t?r, ?zo?r-, ?z?r o??æs t?r, ?zo?r-) n. fl. 6th century B.C., Persian religious teacher. Also called Zarathustra. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.