Why is Spanish sometimes referred to as Castilian or castellano?
            An answer to that question requires a brief look at how the Spanish
            language developed to its current form. What we know as Spanish is
            primarily a derivative of Latin, which arrived on the Iberian
            Peninsula (the peninsula that includes Spain and Portugal) around
            2,000 years ago. On the peninsula, Latin adopted some of the
            vocabulary of indigenous languages, becoming Vulgar Latin. The
            peninsula’s variety of Latin became quite well entrenched, and with
            various changes (including the addition of thousands of Arabic
            words), it survived well into the second millennium.
          
 For reasons more political than linguistic, the dialect of Vulgar
            Latin that was common in what is now the north-central portion of
            Spain, which includes Castile, spread throughout the region. In the
            13th century, King Alfonso supported efforts such as the translation
            of historic documents that helped the dialect, known as Castilian,
            become the standard for educated use of the language. He also made
            that dialect the official language for government administration.
          
 As later rulers pushed the Moors out of Spain, they continued to use
            Castilian as the official tongue. Further strengthening Castilian’s
            use as a language for educated people was Arte de la lengua
            castellana by Antonio de Nebrija, what might be called the first
            Spanish-language textbook and one of the first books to
            systematically define the grammar of a European language.
          
 Although Castilian became the primary language of the area now known
            as Spain, its use didn’t eliminate the other Latin-based languages
            in the region. Galician (which has similarities to Portuguese) and
            Catalan (one of the major languages of Europe) continue to be used
            in large numbers today. A non-Latin language, Euskara or Basque, is
            also spoken by a minority.
          
 In a sense, then, these other languages — Galician, Catalan and
            Euskara — are Spanish languages and even have official status in
            their regions, so the term Castilian (and more often its Spanish
            equivalent, castellano) has sometimes been used to differentiate
            that language from the other languages of Spain.
            Today, the term Castilian is used in other ways too. Sometimes it is
            used to distinguish the north-central standard of Spanish from
            regional variations such as Andalusian (used in southern Spain).
            Sometimes it is used, not altogether accurately, to distinguish the
            Spanish of Spain from that of Latin America. And sometimes it is
            used simply as a synonym for Spanish, especially when referring to
            the “pure” Spanish promulgated by the Royal Spanish Academy         
           (which  itself preferred the term castellano in its dictionaries until the
            1920s).
            In Spain, a person’s choice of terms to refer to the language
            sometimes can have political implications. In many parts of Latin
            America, the Spanish language is known routinely as castellano
            rather than español.
 
Author; Jorge E. Alatorre