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.
List of countries where Spanish is an official language
Spanish has been spoken in the United States for several centuries in the Southwest and Florida, which were all once part of New Spain. However, today only a tiny minority of Spanish speakers in the US trace their language back to those times; the overwhelming majority of speakers come from recent immigration. Only in northern New Mexico and southern Colorado has Spanish maintained speaking communities uninterruptedly since colonial times. Spanish is the most studied foreign language in United States schools and is spoken as a native tongue by 41 million people, plus an additional 11 million fluent second-language speakers. Though not official, Spanish has a special status for education in the U.S. state of New Mexico. With over 50 million native speakers and second language speakers, the United States now has the second largest Spanish-speaking population in the world after Mexico. Spanish is increasingly used alongside English nationwide in business and politics. In the United States, the language is regulated by the North American Academy of the Spanish Language.