A·li·ák·mon (äl-yäk?môn, ä?l?-äk?-) A river, about 320 km (200 mi) long, of northern Greece. It is the longest river in the country.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.