Español o Castellano?
Spanish, Castilian, what’s the difference?
Spanish students will often express their confusion in regard to these two words. Some tend to believe that Castellano implies a regional dialect, while Español refers to the standard usage of the language.
In actual fact, the difference lies in politics rather than linguistics. The term Castellano derives from the Castilla area in Spain, where this ‘language’ was first spoken, just like Catalán, Vasco or Gallego in different parts of the country. However, unlike these other languages, Castellano extended over the rest of the peninsula. The reason was simply the socio-economic importance given to the term by the region in which it was first spoken. Thus Castellano isn’t merely a dialect, but another word for what non-native speakers would collectively refer to as Spanish.
Usually, speakers of Catalán, Vasco or Gallego are unwilling to accept the name Español for their mother tongue, as to them it would imply they were undermining their own language. Nevertheless, as the language has since become an extremely well-represented international language, in countries outside Spain and Latin America the language is mostly referred to as Spanish.
If you’re still confused, Wikipedia attempts to explain the phenomenon thusly:
To understand how two terms can refer to the same language, imagine that the English language were sometimes called English after the historical nation whose language it is, but also sometimes British after the modern state, the United Kingdom of Great Britain and Northern Ireland (UK), of which it is the official language. To add to the complexity, former British colonies such as British North America had to choose a name for the language, as did the speakers of Welsh and other non-English languages in the United Kingdom. This resembles the situation with Spain and its historical centre, Castile.