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.
Galician language
Galician (, ; ) is an Indo-European language of the Western Ibero-Romance branch. It is spoken by some 2.4 million people, mainly in Galicia, an autonomous community located in northwestern Spain, where it is official along with Spanish. The language is also spoken in some border zones of the neighbouring Spanish regions of Asturias and Castile and León, as well as by Galician migrant communities in the rest of Spain, in Latin America, the United States, Switzerland and elsewhere in Europe.