I·shim (?-sh?m?) A river, about 1,820 km (1,130 mi) long, rising in the steppe region of Kazakhstan and flowing generally west then northeast to the Irtysh River in south-central Russia.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.