The
Basque national
flag. It was designed in the late nineteenth century by the nationalist
Sabino Arana, who intended it to be the flag of
Bizkaia province, but it was quickly taken up by the whole Basque Land. Its design is based on the
Union Jack: It has a white cross over a red background, with a green
saltire (diagonal cross) underneath the white one (hidden by it at the centre).
The name ikurrin is also an invention of Arana's, who tinkered with the language in somewhat ill-grounded ways. it ultimately derives from irakurri 'to read', via a sense of 'symbol', and he meant it as the word for 'flag'. But it was adopted as the word specifically for his Basque flag. With the article attached, it is ikurriña or ikurrina, the form it's usually cited in outside the Basque Country.
It is both the official flag of the Basque government (Eusko Jaurlaritza) in its three provinces, and unofficially that of the whole people in all seven provinces in Spain and France.