The names in Dutch and English are similar; this was the office of the chief executive of the Dutch Republic when Holland was a republic and not, as it is now, a Monarchy. Once, the Stadholder had been the King’s representative (and military commander) in any one of the many districts.
When the northern part of the Netherlands, also called the Low Countries because most of the land was below sea level, broke formally (and finally violently) with the Spanish king in 1581 the provincial assemblies or stads decided that they would appoint their own Stadholder.
Sadly, this was not quite as successful as had been hoped, because the position became ambivalent: he was appointed by the district, but he also had the right to appoint officials, who often included members of the states themselves. This invited corruption.
Every province could appoint its own Stadholder, but this was in theory only; in practice the office, which carried much power and opened the door to wealth, went to a member of the House of Orange-Nassau (q.v.).
Actually the Stadholder was not royal, but many of them were successful; Frederick William, for example, and both Williams II and III at least had the ambience of royalty and were good soldiers. Men like Johan de Witt were for ever trying to decrease the power of the Stadholder, and there were periods during which no Stadholder ruled at all in the majority of districts. These were 1670-72 and 1792-47. The wily William III ensured a shaky kind of perpetuity by making the office hereditary. The son of the last of the Stadholders, who was William V – became King William I.
William III of Orange (1650 – 1702) became King of Great Britain as well, by the double virtues of being the son of William II of Orange by Mary, eldest daughter of Charles I of England; and by marrying his English cousin Mary, a daughter of the English king James II (thrown out but never officially abdicated).
Here is the royal House of Orange-Nassau:
1572-84 William I (The Silent)
1791-95 William V
1815-40 William I
Juliana (1980, abd. in favour of . . .)
Beatrix (who abdicates in favour of her son)