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.
Asturian language
Asturian (; "asturianu" ,Art. 1 de la Ley 1/1998, de 23 de marzo, de uso y promoción del bable/asturiano/Law 1/93, of March 23, on the Use and Promotion of the Asturian Language formerly also known as "bable" ) is a West Iberian Romance language spoken in Principality of Asturias, Spain. Asturian is part of a wider linguistic group, the Astur-Leonese languages. The number of speakers is estimated at 100,000 (native) and 450,000 (second language). There are three main variants in the Astur-Leonese language family: Western, Central, and Eastern. For historical and demographic reasons, the standard is based on Central Asturian. Asturian has a distinct grammar, dictionary, and orthography. Regulated by the Academy of the Asturian Language, although it is not an official language of Spain it is protected under the Statute of Autonomy and is an elective language in schools.