The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
or Western Roman Empire
The western section of the Roman Empire, first set apart in A.D. 286 by Emperor Diocletian and later (395) formalized after the death of Theodosius I. It comprised Italy, Spain, Gaul, Britain, Illyricum, and northern Africa and lasted until 476.