The real name of the spanish language, in spanish, which is rarely translated as "Castilian". Español is a much more generic term, that means "from Spain", because in Spain we can find various languages, like basque, catalan, galician, and such. Intermixing spanish and english words can cause pain on people that speaks spanish.

Calling castellano the "real" name of the Spanish language is actually not quite accurate, because what the language is called varies by country.

In Spain or Argentina, for example, you will hear it called castellano, but in Mexico you are much more likely to hear español.