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 language in the United States
The United States has forty-one million people aged five or older that speak Spanish at home, making Spanish the second most spoken language of the United States by far. Spanish is the most studied foreign language in United States, with about six million students. With over 50 million native speakers, heritage language speakers and second language speakers, the United States now has the second largest Spanish-speaking population in the world after Mexico, although it is not an official language of the country. About half of all American Spanish speakers also assessed themselves as speaking English "very well" in the 2000 U.S. Census. This percentage increased to 57% in the 2013-2017 American Community Survey. The United States is among the Spanish-speaking countries that has its own Academy of the Spanish Language.