While reading through the book Medallic illustrations of the history of Great Britain and Ireland to the death of George II vol. I, I came across this interesting description.
My question is why William III is referred to as the "British-Batavian Nassau"? I am unfamiliar with both Batavia and Nassau.
William III was a member of the House of Nassau and, as the Prince of Orange a pre-eminent Dutch leader. In 1672 he became a Stadtholder in the Dutch Republic. Following the Glorious Revolution in 1688, he reigned in Britain as the King of England, Scotand, and Ireland.
Note that during this time, in a display of national romanticism, the Dutch people regarded the ancient Germanic tribe of Batavians to be their precursors.
Combine his position over the supposed Batavi nation (as stadtholder) and the British people (as king) with his Nassau lineage results in NASSAVIUS BRITAN . BATAV., i.e. "British-Batavian Nassau".
William III was of the Dutch house of Orange-Nassau. The name “Batavia” comes from the Germanic tribe of the Batavians, living near the mouth of the Rhine during the Roman republic. The name “Batavia” was also used later, e.g. for the Batavian Republic, in a similar fashion to how “Gallia” and derivations thereof can sometimes be used instead of “France” or “French”.
Why William’s Dutch heritage was highlighted in this way, I cannot answer.
William III was King of England because of his marriage to Queen Mary II of England, who shared power with him in a so-called "Crown Matrimonial." But he had earned this privilege by throwing the Dutch army on the side of the "rebels" against King James II of England (Mary's father).
William's real power base was as the Prince of Orange, known on the "other side" as the house of Nassau, having inherited this position as a descendant of William the Silent, the first "king" of the Netherlands. During the early 18th century, the Dutch considered themselves descended from the Batavians, a Roman-era people who had inhabited the Netherlands.
William added this adjective to his title to highlight his "Dutchness," because Nassau was actually a German place and title. Another way of reading his full title was as the "British-Dutch" German noble.
This particular wording is being chosen to designate which particular sovereign prince of the House of Nassau is being referred to, there being a particularly large and confusing number in 1698 due to family holdings being divided amongst sons of Count Louis II of Nassau-Weilburg (1627) and Count John VI of Nassau-Dillenburg (1606) :
Count of Nassau-Weilburg (Younger) (partitioned in 1627 into the above, Principality from 1688)
Principality of Nassau-Hadamar (County until 1650)
Consequently, when in 1698 one referred to a sovereign member of the House of Nassau one had to be specific as to which one was intended. The particular usage in your text, "British-Batavian Nassau" is probably being used to designate the territories ruled by William III, in line with how other branches of the family were designated.
The nomenclature House of Orange-Nassau, or Prince of Orange would have failed as a description, as the Principality of Orange had been annexed by Louis XIV a few years earlier in 1673. As William III ruled Luxembourg as well as Britain and Netherlands at this time, a reference to Dutch would likewise have failed descriptively. Thus the use of British-Batavian in this context both achieves a pleasant alliteration in English and captures the extent of the territory over which William reigned as one of the most powerful sovereigns in Europe.
As a side note, the city of Djakarta in Indonesia (Dutch East Indies) was named Batavia during Dutch rule
