Most likely you have already heard both words being used as synonyms and asked if there is any difference between them. The answer is that, in practice, both “Spanish” and “Castilian” refer to the same language, and the vocabulary, grammatical rules and spelling are the same.
Now the history of the words is different: “Castilian” appeared first and goes back to the Kingdom of Castile which ruled over the remaining of the territory of current Spain in the Middle Ages. When the State was unified, the Castilian leadership caused the adoption of Castilian as the official language of the new country – and the word “Spanish” came forth.
With the Great Navigations and the colonization of several territories by the Spaniards, the language was chiefly disseminated in Latin America. Thus, in several countries of South America, the word “Spanish” relates to the colonial period; thence, it is more common that the population of these places refers to the language as “Castilian”. This is the case of Argentina, for example; in Mexico, Caribbean and other areas closer to English-speaking countries, the use of “Spanish” is more common.
Today, Castilian is Spain’s official language according to the 1978 Constitution. However, it exists in parallel with other dialects such as Galician, Basque and Catalan which are spoken daily in the country different regions. The manner in which the population refers to the language also varies: in the North, the prevailing word is “Castilian”, and in Andalusia and Canary Islands, “Spanish” is more common.
Although there are no practical differences between “Castilian” and “Spanish”, the language undergoes variations according to the location where it is spoken. Let’s think, for instance, in the differences between Brazilian Portuguese and the language in Portugal: the language is the same, but there are several subtle differences in the use of words, syntactic structures and sounds that cause a message to sound more natural to the population of a specific territory. Therefore, when translating a text, it is essential to bear in mind the final reader; that’s why it is so important that translators should be knowledgeable in both source and destination languages, as well as the specificities of each public so that the message would be well conveyed and clearly understood by the target public.