Arrano beltza

The arrano beltza ("" in Basque) is an ancient Basque and Navarre symbol which shows a black eagle upon a yellow background. Today it is mostly, though not exclusively, used by Basque nationalists as a symbol of Euskal Herria.

The black eagle was originally the seal of King Sancho VII of Navarre but was later attributed to Sancho III of Navarre who, when incorporating Aragon and Castile, had under his crown all the territories of Basque culture and language, including those traditionally Castilian, since his kingdom reached from Galicia to Ribagorza. From a Basque nationalist interpretation, the rule of Sancho III constitutes a historical precedent for the aspirations of the unification of the Basque-speaking territories under one independent State. From a Spanish perspective, it is one of the first attempts at forming a unified Spain.

Note that the flag is a modern interpretation of the seal. There are no known flags prior to the 18th century when Father Isla describes a crimson flag with the chains and crown of the modern coat-of-arms.

This symbol is used mostly by the so-called Ezker Abertzalea (Nationalist Left), who consider it "the oldest symbol of the territory of Euskal Herria". Such use fits in with the search for symbols and terms which go beyond the iconography created by Sabino Arana. Thus, besides the ikurriña (the official flag in the Basque Autonomous Community), of Aranist origin, one can find the yellow flag with the black eagle and the flag of Navarre being used more often, as they are considered more genuine symbols of Euskal Herria. For the same reason, these leftist abertzaleak often use the traditional term Euskal Herria instead of the neologism Euskadi, created by Sabino Arana in the 19th century. (Euskadi, on the other hand, is starting to be more usually identified with the Basque Autonomous Community.)

...
Wikipedia