Akelarre is the Basque term meaning Witches' Sabbath (the place where witches hold their meetings). Akerra means male goat in the Basque language. Witches' sabbaths were envisioned as presided over by a goat.
The word has been loaned to Castilian Spanish (which uses the spelling Aquelarre). It has been used in Castilian Spanish since the witch trials of the 17th century. The word is most famous as the title of the witchcraft painting by Goya in the Museo del Prado, which depicts witches in the company of a huge male goat.