Biscayan, sometimes Bizkaian (Basque: Bizkaiera), is a dialect of the Basque language spoken mainly in Biscay, one of the provinces of the Basque Country of Spain.
It is the only member of the western branch of the Basque language, with the dialect being named as Western in the classification drawn up by linguist Koldo Zuazo, since it extends slightly into the fringes of Alava and deeper in the west region of Gipuzkoa. The dialect's territory bears great similarity to that of the Caristii tribe, as described by Roman authors.
A perceived linguistic trait differing from other dialects is the form of the auxiliary verb, namely the form "eutsi" or the conspicuous absence of past tense 3rd person mark z- at the beginning of auxiliary verbs. While it is treated as stylish to write in Biscayan, and it is still spoken generally in about half of Bizkaia and some other municipalities, the dialect suffers from the double pressure of Unified Basque and Spanish.