In English, Castilian Spanish can mean the variety of Peninsular Spanish spoken in northern and central Spain, the standard form of Spanish, or Spanish from Spain in general. In Spanish, the term castellano (Castilian) can either refer to the Spanish language as a whole, or to the medieval Old Spanish, a predecessor to Early Modern Spanish.
See also: Names given to the Spanish language
The term Castilian Spanish is used in English for the specific varieties of Spanish spoken in north and central Spain. This is because much of the variation in Peninsular Spanish is between north and south, often imagined as Castilian versus Andalusian. Typically, it is more loosely used to denote the Spanish spoken in all of Spain as compared to Spanish spoken in Latin America. In Spain itself, Spanish is not a uniform language and there exist several different varieties of Spanish; in addition, there are other official and unofficial languages in the country, although Spanish is official throughout Spain.
Castellano septentrional ("Northern Castilian") is the Spanish term for the dialects from the Northern half of Spain, including those from Aragón or Navarre, which were never part of Castile. These dialects can be distinguished from the southern varieties of Andalusia, Extremadura, and Murcia. Español castellano, the literal translation of Castilian Spanish, is not a common expression; it could refer to varieties found in the region of Castile; however, the dialects of Castile, like most dialects, are not homogenous, and they tend to merge gradually with the dialects of other regions.