Castilian (Castellano) is another name for Spanish — specifically the Spanish dialect of central Spain that became the standard form of the language. Worldwide it's interchangeable with "español" in most contexts, though within Spain "castellano" is sometimes preferred to distinguish Spanish from other co-official languages (Catalan, Basque, Galician). For US translation work, a request for "Castilian" usually signals a preference for European Spanish over Latin American Spanish — relevant for documents going to or from Spain, particularly civil records and notarised translations for Spanish dual citizenship.