The Basque Country isn’t as difficult to define as the whole thing with the Netherlands and Holland, but almost. One of Spain’s 17 autonomous communities is called the Basque Country and consists of the three provinces Araba/Álava, Bizkaia and Gipuzkoa in the north of the country.
However, the Basque Country can also refer to a greater region, also including the neighbouring province Navarre and three historical pronvinces in the southwest of France: Lapurdi, Lower Navarre and Zuberoa. This greater region is depicted in the map in this mural from Deba, calling for “freedom for the Basque Country” and showing the red, white and green Basque flag (even though the flag is only officially used in the autonomous community, the western half of the map):
The names of the region in French, Pays basque, and in Spanish, País vasco, are equivalent to the English term. In Basque however, there are two different names: Euskadi and Euskal Herria. There’s a tendency that Euskadi (coined in the 19th century) is more often used to refer to the autonomous community, while Euskal Herria (dating from at least the 16th century) is used for the greater region, but there are also examples of the opposite, so you never really know. Read more at Wikipedia and Eusko Guide.
References to the greater region can also be found in graffiti. For example, this painting in Getaria shows the heraldic shields of the six/seven provinces (the lower symbol in the middle represents both (Upper) Navarre and Lower Navarre), with the headline Euskal Herria:
A more colourful version of the Navarrese shield can be seen in this detail of a mural in Azkoitia, Gipuzkoa:
The symbol with the six shields isn’t officially recognised (the Basque Autonomous Community has another heraldic shield while the Chartered Community of Navarre has its own, kind of like the one above but with a crown added to it). However, sometimes it is also used in more offical context, for example at the facade of the local food market in Zarautz, Gipuzkoa, where the shield of the municipality is painted to the left, and the shield of Euskal Herria is to the right: