A Palatine was an inhabitant of the Palatinate (German Pfalz), the domain of a Count Palatine in the Holy Roman Empire. By the 14th century, this title had become hereditary and was held by the Wittelsbach family. The Palatinate had become the leading state of the Empire and the Count Palatine one of the Electors who chose the Emperor.
The exact boundaries of the Palatinate varied as political and military fortunes of the Wittelsbachs ebbed and flowed but, by the 17th century, the Palatinate was composed primarily of two large, distinct areas. The Lower Palatinate, also known as the Rhenish Palatinate (Rheinpfalz), was in the southwestern portion of Germany. It stretched from the Moselle River down to the borders of Alsace, eastward below the Neckar River out past Heidelberg (the capital in those days), north over through Mainz up toward Koblenz. Today, this area is largely in the state of Rheinland-Palatinate. The Upper Palatinate, or Oberpfalz, was located in the northeastern part of Bavaria, starting around the Naab River and stretching over to the Bohemian forests. Today, this area is contained with in the state of Bavaria.
In 1517, Martin Luther touched off the Reformation and, in the 1560s, the Palatinate converted to the Protestant faith under Elector Frederick III. It became a center for Calvinism, attracting individuals of that creed from all over central Europe. However, the 17th century brought a host of troubles to the Palatinate from which it never really recovered, and which precipated the large-scale emigration of its inhabitants...many to the New World.
The Habsburgs, who had come to control the nominally elected Imperial throne, were staunchly Catholic. When Emperor Rudolf II and his brother, Matthias, died without heirs, the the governance fell to Archduke Ferdinand (later Ferdinand II). He was a resolute counter-reformer and immediately started curtailing religious freedom—notably in Bohemia, which had won an Imperial guarantee of religious freedom in 1609. In response, the Bohemians offered their crown to Elector Frederick V of the Palatinate...and he accepted. This triggered the Thirty Years War, pitting the largely Catholic south of Germany against the largely Lutheran north, Catholic Spain against Protestant Netherlands, with France working to increase its territories wherever possible. The Palatinate, in the center, went from the leading state of Germany to a ravaged spoil fought over by all. By the time peace was nominally declared in 1648, millions were dead and the town and cities of the Palatinate had been demolished.
Louis XIV of France, continuing his wars with the Netherlands, did not give up his intentions of expanding his borders in all directions and French troops continued to pillage the Palatinate for the coming decades. In the mid-1670s, the French devasted the Palatinate in a retreat. In response, Germany reduced the Rhineland to a wasteland so that a returning French army would find no subsistence—every field was burned, towns razed, castles destroyed. In 1698, the region was once more formally at war as the German princes joined with England and Spain to fight France in the War of the Grand Alliance (also known as the War of the Palatinate Succession).
Finally, as a fitting end to this period, the winter of 1708 was the harshest in 100 years. The Rhine froze; the grapevines were killed; the cattle died in the barns.
In 1709, partly at the behest of Queen Anne, who was related to the Elector and whose husband was a Lutheran, the British passed a naturalization act whereby any Protestant willing to swear an oath of allegiance to the British government would immediately acquire the rights and privileges of an English subject and the Palatine emigration started. That spring, approximately 7,000 individuals fled the Palatinate to Rotterdam. About half were sent to the New York area in America as a buffer against the French Canadians, the remainder heading to Ireland to bolster the Protestant settlements there. The rush continued through the year, though Britain tried to reverse its policies, unable to cope. By the end of September there were 13,000 Germans in London awaiting transfer, most in tents outside the city.
Though Britain stopped its settlements of the New York area, William Penn had opened Pennsylvania to Swiss, Germans and Dutch settlers. Those who made their way their encouraged others to follow them and common estimates are that 15,000 reached Pennsylvania by 1727 and 75,000 by 1750.
As a final note, some sites state that religious persecution was a major factor in the early Palatine emigation. I don't find this convincing. Though Elector John William, ruler from 1690 to 1716, was a devout Catholic, there is only a small record of religious persecution, early in his reign, and most of it directed at Protestants other than Lutherans and Reformed, which were officially recognized religions of Germany. Yet, the numbers in the 1709 treatise "A Brief History of the Poor Palatine Refugees Lately Arrived in England" shows that the arriving immigrants were split almost equally between Lutherans, Calvinists and Catholics, none of whom made any reports of persecution.