You will probably find many diverse and conflicting answers in this regard.  However, the correct answer is that both español and castellano synonymously translate to Spanish, referring to the language spoken in Spain, in many American countries (Latin American and the USA) and in other parts of the world. There is a more appropriate term used to refer to Spanish (as a unified language) and that term is español, as it is all-encompassing and unambiguous. Moreover, it is the term used internationally to refer to the Spanish language, like in these examples: Spanish, espagnol, Spanisch, spagnolo, etc. The term castellano (Castillian Spanish), on the other hand, although in current day Spanish is still synonymous with the term español, is preferably used to distinctly refer to: - the Romance dialect born in the Kingdom of Castilla in the Middle Ages; - the Spanish dialect actually spoken in the region of Castilla, Spain: or - the common language of the Spanish state, together with its other co-official languages such as Catalán, Basque or Gallego.