Names given to the Spanish language
Formally speaking, the national language of Spain, the official Spanish language, is the Castilian language (as opposed to regional Spanish languages like Galician, Catalan, Asturleonese and Basque). As such both names, "español" and "castellano", have distinct and independent meanings that may be required for clarity in some specific contexts.
Generally speaking, though, both terms can be used to refer to the (national) Spanish language as a whole, with a preference for one over the other that depends on the context or the speaker's origin. "Castellano" (as well as "Castilian" in English) has another, more restricted, meaning, relating either to the old Romance language spoken in the Kingdom of Castile in the Middle Ages, predecessor of the modern Spanish language, or to the variety of Spanish nowadays spoken in the historical region of Castile, in central Spain.
Spanish Empire
The Spanish Empire left a huge linguistic, religious, political, cultural, and urban architectural legacy in the Western Hemisphere. With over 470 million native speakers today, Spanish is the second most spoken native language in the world, as result of the introduction of the language of Castile—Castilian, ""Castellano"" —from Iberia to Spanish America, later expanded by the governments of successor independent republics. In the Philippines, the Spanish–American War (1898) brought the islands under U.S. jurisdiction, with English being imposed in schools and Spanish becoming a secondary official language.