Spanish (español) sometimes called Castilian (castellano) is a Romance language that originated in northern Spain, and gradually spread in the Kingdom of Castile and evolved into the principal language of government and trade. It was taken most notably to the Americas, and also to Africa and Asia Pacific with the expansion of the Spanish Empire between the fifteenth and nineteenth centuries.

In contemporary Spain, Castilian is just one of several regional languages spoken including Aranese, Basque, Catalan/Valencian and Galician. Today, about 350 million people speak Spanish as a native language, making it the world's second most spoken language in terms of native speakers but fourth in terms of total speakers. Mexico contains the largest population of Spanish speakers.