Noble titles: Prince
The original, but now less common use of the word, originated in the application of the Latin word princeps, from Roman, more precisely Byzantine law, and the classical system of government that was the European feudal society. In this sense, a prince is a ruler of a territory which is sovereign, or quasi-sovereign, i.e., exercising substantial (though not all) prerogatives associated with monarchs of independent nations, as was common, for instance, within the historical boundaries of the Holy Roman Empire. In medieval and Early Modern Europe, there were as many as two hundred such territories, especially in Italy and Germany. In this sense, "prince" is used of any and all rulers, regardless of actual title or precise rank. This is the Renaissance use of the term found in Niccolò Machiavelli famous work, Il Principe.
As a title, by the end of the medieval era, prince was borne by rulers of territories that were either substantially smaller than or exercised fewer of the rights of sovereignty than did emperors and kings. A lord of even a quite small territory might come to be referred to as a prince before the 13th century, either from translations of a native title into the Latin princeps (as for the hereditary ruler of Wales), or when the lord's territory was allodial. The lord of an allodium owned his lands and exercised prerogatives over the subjects in his territory absolutely, owing no feudal homage or duty as a vassal to a liege lord, nor being subject to any higher jurisdiction. Most small territories designated as principalities during feudal eras were allodial, e.g. the Princedom of Dombes.
Lords who exercised lawful authority over territories and people within a feudal hierarchy were also sometimes regarded as princes in the general sense, especially if they held the rank of count or higher. This is attested in some surviving styles for e.g., British earls, marquesses, and dukes are still addressed by the Crown on ceremonial occasions as high and noble princes (cf. Royal and noble styles).
In parts of the Holy Roman Empire in which primogeniture did not prevail (i.e. Germany), all legitimate agnates had an equal right to the family's hereditary titles. While this meant that offices, such as emperor, king, and elector could only be legally occupied by one dynast at a time, holders of such other titles as duke, margrave, landgrave, count palatine, and prince could only differentiate themselves by adding the name of their appanage to the family's original title. Not only did this tend to proliferate unwieldy titles (e.g. Princess Katherine of Anhalt-Zerbst ane Karl, Count Palatine of Zweibrucken-Neukastell-Kleeburg), but as agnatic primogeniture gradually became the norm in the Holy Roman Empire by the end of the eighteenth century, another means of distinguishing the monarch from other members of his dynasty became necessary. Gradual substitution of the title of Prinz for the monarch's title of Fürst occurred, and became customary in all German dynasties except in the grand duchies of Mecklenburg and Oldenburg. Both Prinz and Fürst are translated into English as "prince", but they reflect not only different but mutually exclusive terms.
This distinction had evolved before the eighteenth century (in most families: Liechtenstein long remained an exception, cadets and females using Fürst/Fürstin into the 19th century) for dynasties headed by a Fürst in Germany. The custom spread through the Continent to such an extent that a renowned imperial general who belonged to a cadet branch of a reigning ducal family, remains best known to history by the generic dynastic title, Prince Eugene of Savoy. Note that the princely title was used as a prefix to his Christian name, which also became customary.
Cadets of France's princes étrangers began to affect similar usage but when, for example, the House of La Tour d'Auvergne's ruling dukes of Bouillon, attempted to use the same style, it was initially resisted by historians such as Père Anselme -- who, however, willingly recognized use of territorial titles, i.e. he accepts that the ducal heir apparent is known as prince de Bouillon, but would record in 1728 only that the heir's cousin, the comte d'Oliergues was "known as the Prince Frederick" ("dit le prince Frédéric").
The post-medieval rank of gefürsteter Graf (princely count) embraced but elevated the German equivalent of the intermediate French, English and Spanish nobles. In Germany, these nobles rose to dynastic status by preserving from the Imperial crown (de jure after the Peace of Westphalia in 1648) the exercise of such sovereign prerogatives as the minting of money; the muster of military troops and the right to wage war and contract treaties; local judicial authority and constabular enforcement; and the habit of inter-marrying with sovereign dynasties. Eventually, these titles came to be more highly valued than that of Fürst itself, and by the 19th century, their cadets would become known as Prinzen.