Val·en·tin·ian I 
(văl′ən-tĭnē-ən, -tĭnyən) AD 321-375.
(văl′ən-tĭnē-ən, -tĭnyən) AD 321-375. Emperor of Rome in the West (364-375) who ruled jointly with his brother Valens in the East. He led numerous military campaigns in Africa and Europe.