Castilian is the official language of Spain and all the countries that have been colonized by Spain (Almost all the Latin American countries). The term 'Spanish' is a slip-up. In Castilian itself, it is named "Castellano". It is the same language spoken in Spain (just different accent). You oughtn't forget that 4 languages are spoken in Spain (Spanish languages) which are Castilian (Official one) Galician, Catalan, and Basque.
Ezekiel: Hey, Paul, how are you doing?
Paul: Hi, Zek, I'm pretty good, thanks!
Ezekiel: why are you so nervous?
Paul: I'm not nervous, I'm just in a hurry... I have to go to my Castilian Class... sorry but I have an exam tomorrow and I have my books of Castilian in my house!! Bye!