Which are the words, parts of words and structures coming from Basque to Spanish language? And possibly in which periods did they become part of spoken Spanish and official Spanish (Castellano)?
From the top of my head, the most used one would be
There are others, like:
The DRAE attributes a Basque origin to the following words:
It attributes a possible Basque origin to the following:
The word agur was borrowed from Basque, which in turn had borrowed it from Latin.
The word silueta comes from a fellow named Étienne de Silhouette, whose surname comes from the Basque zuloeta, which is related to the zulo mentioned by Mikmik in a previous response.
This list isn't comprehensive, of course. I didn't include words mentioned in previous responses, recent borrowings, or regionalisms.