San Di·e·go (sān dē-ā'gō) A city of southern California on San Diego Bay, an inlet of the Pacific Ocean near the Mexican border. Although explored by the Spanish in 1542, the area's first permanent settlements were not established until the 1700s. A noted zoological park is located in the city, which is a major port of entry and a commercial, industrial, and research center. Population: 1,260,000.