Akelarre is the Basque term meaning Witches' Sabbath (the place where witches hold their meetings). Akerra means male goat in the Basque language. Witches' sababaths 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.
The most common etymology proposed is that meaning meadow (larre) of the male goat (aker "buck, billy goat"). The Spanish Inquisition accused people of worshipping a black goat, related to the worship of Satan. An alternative explanation could be that it originally was alkelarre, alka being a local name for the herb Dactylis hispanica. In this case, the first etymology would have been a manipulation of the Inquisition, the fact being that the Basques did not know during the 1609-1612 persecution period or later what the "akelarre" referred to by the inquisitors meant. The word "aquelarre" is first attested in 1609 in a Spanish language inquisitorial briefing, as synonym to junta diábolica, meaning 'diabolic assembly'. Basque terms, transcribed into Spanish texts often by monolingual Spanish language copyists, were fraught with mistakes.
Nevertheless, the black he-Goat or Akerbeltz is known in Basque mythology to be an attribute of goddess Mari and is found in a Roman age slab as a votive dedication: Aherbelts Deo ("to the god Aherbelts") (see: Aquitanian language)..