Da·ri·us I (d?-r???s) Known as “Darius the Great.” 550?-486 bc. King of Persia (521-486) who expanded the empire, organized a highly efficient administrative system, and invaded Greece, only to be defeated at the Battle of Marathon in 490.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.Darius I (d??ra??s) n (Biography) known as Darius the Great, surname Hystaspis. ?550?486 bc, king of Persia (521?486), who extended the Persian empire and crushed the revolt of the Ionian city states (500). He led two expeditions against Greece but was defeated at Marathon (490)Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Da?ri?us I (d??ra? ?s) n. (Darius Hystaspes) ( ?the Great? ), 558??486? B.C., king of Persia 521?486. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.