The Spanish language has two names: () and (). Spanish speakers from different countries or backgrounds can show a preference for one term or the other, or use them indiscriminately, but political issues or common usage might lead speakers to prefer one term over the other. This article identifies the differences between those terms, the countries or backgrounds that show a preference for one or the other, and the implications the choice of words might have for a native Spanish speaker.
Formally speaking, the national language of Spain, the official Spanish language, is the Castilian language (as opposed to the regional languages of Spain, such as 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.
History of the terms
Originally Castilian (castellano) referred to the language of the Kingdom of Castile, one of several northern kingdoms that spread across the Iberian Peninsula through the Middle Ages, from about the 8th to the 15th centuries. Traditionally the first recorded examples of written Castilian/Spanish are considered to be the Glosas Emilianenses, a number of isolated words added to a Latin text as an aid to the reader, dated to the eleventh century. Soon after that there begin to appear discursive texts in Castilian, such as the Cantar de Mio Cid. This early Romance language was derived from Latin and evolved into modern Spanish.
However, the term Spanish (español) is a more recent term that first referred to Spain as a country, and then to the predominant language spoken in that country. Spain as a truly unified nation appeared centuries later than the language and the Kingdom of Castile; in fact, it was only in the late 15th century that the personal union between the Crowns of Castile and Aragon unified Spain. The actual legal unification date is disputed, but commonly agreed to have occurred not earlier than the eighteenth century at the end of the War of the Spanish Succession. Only then did the Castilian language begin to be commonly called Spanish.
In 1492, the arrival of Christopher Columbus on a Castilian-paid expedition paved the way for the Spanish colonization of the Americas. As a result of this process, most countries in South America now speak the same language as Castile. Until about the eighteenth century, the Kingdom of Castile, and not Spain as a whole, was the colonizing power, and the language used was called castellano. Thus, some Latin American countries formerly under Spanish rule have retained the custom of calling it castellano, while others eventually switched to calling it español, with many different factors influencing the final choice.
In English, the term Spanish relates both to the language and to the nation. The noun used for a person from Spain is Spaniard, with the collective noun the Spanish. The term Castilian is much less widespread amongst English speakers than the term Spanish.
Español
Two main hypotheses have been proposed for the origin of the word español: one based on dissimilation in Old Spanish españón (from a presumed Vulgar Latin *hispaniōne), and the other on an Occitan term derived from a presumed Vulgar Latin *hispaniolus. Both Latin ancestor words are based on the place name Hispania (which evolved into España by regular sound changes); and both are marked with an asterisk to indicate that they are reconstructed, not directly attested.
The dissimilation hypothesis, advanced by Ramón Menéndez Pidal, presumes that Latin Hispania was lengthened by the derivational suffix -ōne (which survives in other ethnonyms such as bretón, borgoñón, sajón, and lapón). The Old Spanish form españon is documented in works of the 13th and 14th centuries. It is suggested that the final /n/ of this form changed to /l/ by dissimilation from the previous nasal consonant, ñ. This sporadic sound change is observed in some other words: Menéndez Pidal cites Barcelona (from Barcinone) and delante (from de in ante); Lathrop adds ingle (from ing[ui]ne) and sangre (from sang[ui]ne).