Names given to the Spanish language
Formally speaking, the national language of Spain, the official Spanish language, is the Castilian language (as opposed to regional Spanish languages like Galician, Catalan, Asturleonese and Basque). As such both names, "español" and "castellano", have distinct and independent meanings that may be required for clarity in some specific contexts.
Generally speaking, though, both terms can be used to refer to the (national) Spanish language as a whole, with a preference for one over the other that depends on the context or the speaker's origin. "Castellano" (as well as "Castilian" in English) has another, more restricted, meaning, relating either to the old Romance language spoken in the Kingdom of Castile in the Middle Ages, predecessor of the modern Spanish language, or to the variety of Spanish nowadays spoken in the historical region of Castile, in central Spain.
Spanish language
In Western Sahara, formerly Spanish Sahara, Spanish was officially spoken during the late nineteenth and twentieth centuries. Today, Spanish in this disputed territory is maintained by populations of Sahrawi nomads numbering about 500,000 people, and is de facto official alongside Arabic in the Sahrawi Arab Democratic Republic, although this entity receives limited international recognition.