Teh·ran or Te·he·ran (t???-r?n?, -rän?, t?-r?n?, -rän?) The capital and largest city of Iran, in the north-central part of the country south of the Caspian Sea. A commercial and industrial center, it became the capital of Persia (Iran) c. 1788 and was modernized under Shah Mohammad Reza Pahlavi (ruled 1941-1979).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.Tehran (t???r??n; -?ræn) or Teherann (Placename) the capital of Iran, at the foot of the Elburz Mountains: built on the site of the ancient capital Ray, destroyed by Mongols in 1220; became capital in the 1790s; three universities. Pop: 7 352 000 (2005 est)Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014