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 no official recognition in the Central American nation of Belize, a Commonwealth realm where English is the official national language. However, the country shares land borders with Spanish-speaking Mexico and Guatemala and, per the 2010 Belizean census, Spanish is spoken by a sizable portion of the population; 30% claim Spanish as a mother tongue and about 50% of the population has working knowledge of the language.