The common name in the 16th AD was
"castellano" as we find in some amerindian languages: nahuatl
(Aztec) <caxtilancopa> 'in Spanish' or Tzotzil (Mayan)
<caxtilan> 'person form Spain'. And traditionally the language was named
<castellano>. It was later when the use of the term "español" spread,
and at the end of 18th it was established "la Real Academia de la Lengua
Española" (Royal Academy of Spanish Language or Spanish Royal Academy of
Language).
Efectively in Catalonia, Galicia and Basque Country
some person by political reason use the term "castellano", but also
non-politiced persons in these areas use the term "español". In English
linguistic writting the term Spanish is more common, and the term Castilian is
used in the sense of "Old Spanish". But for native speakers of most areas
"castellano" or "español" is equally acceptable and none of them is a marked
term.
Davius S.