1. The original name of Spain's official language. Castilian was originally spoken in the County of Castile (Condado de Castilla)and, when Castile got independent from León, this language was spread as Castile conquered most of the Iberian Peninsula, the Canary Islands and part of America.
As the Crown of Castile was much more important, powerful and rich than the Crown of Aragon (Aragon, Valencia, Catalonia and the Balearic Isles), it became Spain's main language.
2. People born in the Spanish regions of Castile-La Mancha (Castilla-La Mancha), Madrid, Cantabria, La Rioja and the provinces of Castile and Leon (Castilla y León) which belonged to the Kingdom of Castile until Castile and Leon rejoined in 1230 (Burgos, Ávila, Segovia, Palencia, Soria and Valladolid)
1. Castilian was formed in the County of Castile.
2. I was born in Cuenca, so I am Castilian.
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!