Spanish Noun: Castellano
Some people in Spain prefer to use this word instead of “Spanish”, when it comes to talk about their language; and this is because there are in the country some bilingual regions, with a language of their own. As ‘Castilla’ prevailed militarily and politically over the other provinces, its language became the official tongue in the country… but the other ones are not forgotten.
In the rest of the world, you can use ‘Español’ or, with more detail: ‘Español castellano’ [Castilian Spanish]. In linguistics, there is yet another denomination: ‘Español de España’ as opposed to ‘Español de México’ or another Latin-America country. This is important for the foreigner, as some “bad words” are normal words in other countries.
Do you speak castilian?