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.
Dominican Spanish
Dominican Spanish, a subset of Caribbean Spanish, is based on the Andalusian and Canarian Spanish dialects of southern Spain, and has borrowed vocabulary from Taíno and other Arawakan languages, from West African languages, and from English. Speakers of Dominican Spanish may also use conservative words that in the upper-class speech of Spain would be considered archaisms. The variety spoken in the Cibao region is a mixture of two dialects: that of the 16th- and 17th-century Portuguese colonists in the Cibao valley, and that of the 18th-century Canarian settlers. According to the linguist John Lipski, the greatest extra-Hispanic influences on Dominican Spanish have been Haitian Creole and West African languages.