Prince

Prince
The term prince (the female form is princess), from the Latin root princeps, when used for a member of the highest aristocracy, has several fundamentally different meanings - one generic, and several types of titles.

Historical background and the two main notions of princehoodThe Latin word Princeps, kin to "primus" and "first among equals", was established as the title of the more or less informal leader of the senate some centuries BCE. Emperor Augustus established the formal position of monarch on basis of principate, not dominion. He also tasked his grandsons as summer rulers of the city when most of the government were on holiday in country or making religious rituals, and for that task, granted the title Princeps.
In Latin-based languages, Prince has two basic meanings: it could be a substantive title and a courtesy title. Substantive princes are in some cases reigning monarchs, and in some cases heads of their noble house. Courtesy princes may be members of a royal or a highly noble family, sharing their title with several relatives in similar position. Many other languages have (at least) two separate words for these two distinct meanings.
Abstract notionThe original but least common use is as a generic (descriptive, not formal) term, one originating in the application of terminology from Roman (actually Byzantine) law and classical "ideology" to the European feudal society. In this sense, it can in principle be used for any ruling (hereditary or elective) monarch, regardless of his title and protocolary rank.
Example: The early Renaissance title of Niccolò Machiavelli's book Il Principe (The Prince) refers to this meaning of prince. The following parts of this article are only concerned with the use usages as a formal nobiliary (or analogous) title.
Genealogical Princes, by birth or equivalentA Prince of the blood (in some monarchies, however, this is an actual title in its own right, of more restricted use; thus Prince du sang in the French kingdom, restricted to the royal descendents in the male line) is a male member of royalty, i.e. of a princely house, such as an imperial - or royal family. Depending on individual national tradition, this may either be restricted (often to one or two generations after the monarch, and/or the line of succession), or it may be allowed to run into very high numbers (as often applies in oriental dynasties).
Generally, when such a prince takes a (royal, imperial, etc.) throne he stops being styled a mere "Prince" when he becomes the ruling (or at least titular) monarch, King, Emperor, Grand Duke or one of many other ruler-styles, usually of higher rank, except in the case of a ruler styled "Prince" (see below) of a principality (idem: "Princess" becoming a Queen).
The female form is "princess", but this is also generally used for the spouse of any Prince (of the blood, or of a principality), and also the daughter of any monarch, though in some monarchies (by law and/or tradition) the award is explicit, not automatic. Inversely, the husband of a born princess is (or was) in many monarchies not as readily styled prince (although it certainly occasionally happened). To complicate matters, the style Royal Highness, normally accompanying the title "Prince" in a dynasty (if of royal or imperial rank, that is), can be awarded separately (as a compromise or consolation prize, in some sense). Regardless of birth rank, marriage to a prince(ss) generally means accession to the ruling house (dynasty), but often the princely style is subject to an explicit conferral (by the Monarch or a political authority with in say in the succession, e.g. certain parliaments), which me be delayed, withheld or even reversed.
In these systems, a prince can be:
The son of a monarch and in the direct line of succession. Other members of the royal family, styled a Royal Highness, and also in the order of succession (although more distant). (In constitutional monarchies the precise rules for succession are fixed by law, possibly even the constitution, but may involve parliamentary assent) The husband of a reigning queen is usually titled "prince" or prince consort. However for wives of Monarchs, the title is usually a female variation on his (the same as used in case a female can mount the throne), such as Queen or Empress; but in cultures which, contrary to Christian traditions, allow the ruler to have several wives (e.g. four in Islam) and/or official concubines, for these women (sometimes collectively referred to as harem) there are often specific rules determining their hierarchy and a variety of titles, which may distinguish between those whose offspring can be in line for the succeesion or not, or specifically who is mother to the Heir to the throne (possibly reaching another official position when he succeeds) Although the definition above is the one that is most commonly understood, there are also different systems: depending on country, epoch and translation other meanings of "Prince" are possible. Over the centuries foreign-language titles such as Italian principe, French prince, German Fürst, Russian kniaz, etc., are often rendered as "prince" in English.
Many princely styles and titles are used in various monarchies, often changing with a new dynasty, even altered during one's rule, especially in conjunction with the style of the ruler. Indeed, various princely titles are derived from the ruler's, such as (e)mirza(da), khanzada, nawabzada, sahibzada, shahzada (all using the Persian patronymic suffix -zada, "son, descendant", (maha)rajkumar from Raja, or even from a unique title, e.g. mehtarjao. However, often such style is used in a way that may surprise as not apparently logical, such as adopting a style for princes of the blood which is not pegged to the ruler's title, but rather continues an old tradition, asserts genealogical descendency from and/or claim of political succession to a more lofty monarchy, or simply is assumed 'because we can'.
In some monarchic dynasties, a very specific title is used, some official, such as Infante in Iberia. This can be a style in existence for a 'princely' -at least originally- feudal entity, possibly still nominally linked to one, Archduke in the Habsburg empire, Grand Prince (often rendered, less correctly, as Grand Duke) in tsarist Russia; see also Porphyrogenetos. On the other hand, an existing style can be used without retaining any of its intrinsic qualities, e.g. Sultan for ordinary members of the Ottoman dynasty (ruler mainly styled Padishah) Sometimes a specific title is commonly used in a region, e.g. Mian in various of the Punjabi princely Hill States (lower Himalayan region in British India) Other titles are unique to one dynasty, even tough the ruler's titel isn't, such as Moulay (French form; also Mulay in English) in the Sherifian sultanate (now kingdom ruled by a Malik) of Morocco, Some monarchies also commonly awarded somo of their princes of the blood various lofty titles, some of which were reserved for royalty, other also open to the most trusted commoners and/or the highest nobility, as in the Byzantine empire (e.g. Protosebastos reserved). For the often specific terminology concerning a probable future successor, see Crown Prince and links there.