Aq·ui·ta·ni·a (?k?w?-t??n?-?) A Roman division of southwest Gaul extending from the Pyrenees to the Garonne River and roughly coextensive with the historical region of Aquitaine. Its Iberian peoples were conquered by Julius Caesar in 56 bc. The region passed to the Franks in ad 507.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.