When did prince/princess come to mean "royal heir"?

Question

The words prince and princess come to English from Old French and ultimately from Latin's "princeps". However, in both Latin and Old French, as well as historical Italian, "prince" refers to the ruler of a country or province (e.g. Machiavelli's The Prince). Often Prince was historically used as a catch-all term to cover sovereign kings, dukes, counts, jarls, and archbishops regardless of title.

Yet somehow between the 17th century and the 20th, in British and American English, prince came to primarily refer to royal heirs, rather than the rulers themselves. How/why did this transformation happen?

Answer 1

OED explains that the royal heir sense is originally in Prince of Wales, a title of the deposed Welsh rulers conferred (from the 14th cent. on) upon the eldest surviving son of the King or Queen of England, the epithet prince being later extended to all male children of the reigning British royal family and, eventually, those of other countries. OED also adds that it may be influenced by the earlier usage in Roman empire for Augustus and his successors. Here is the full etymology of this sense from OED:

Sense 7 originated in the title Prince of Wales , which originally was simply a continuation of sense 6, as title of the deposed native Welsh rulers; but was from the reign of Edward III onwards customarily conferred upon the eldest surviving son of the King or Queen of England, and so came to be applied to this relationship (compare discussion at Welsh adj.). The Prince of Wales was at first the only ‘prince’ in England (see quot. 1577 at sense 7); the title Prince of Scotland was, however, used from the 15th cent. to the Union of 1707 for the eldest son of the King of Scotland (thereafter being used only technically in relation to the superiority of lands in the Principality: see quot. 1861 at sense 7). During the reign of James I and VI of Scotland, the general appellation ‘prince’ was extended to all the sons of the sovereign, and under Victoria (with ‘princess’) to all the grandchildren, being children of sons (quot. 1885 at sense 7). The equivalent of ‘prince’ has been given, usually with some modifying word, to the heir apparent to the throne in various countries; the earliest such examples appear to be Spanish Príncipe de Asturias , lit. ‘Prince of Asturias’ (early 15th cent. or earlier; apparently after English Prince of Wales ) and Middle French prince roial , Middle French, French prince royal prince royal n. Later parallels include German Kronprinz (17th cent.), Swedish kronprins (c1700; after German), Danish kronprins (end of the 17th cent.; probably after German), all lit. ‘crown prince’, and French prince impérial denoting the heir apparent in the French Empire of 1852–70 (1857 or earlier). In most of these countries the title of prince has also been given to male members of the reigning family. This sense may have been partly influenced by Roman usage under the empire, in which the title classical Latin princeps iuventūtis ‘chief’ or ‘prince of the youth’, which was bestowed by the Equites upon the two grandsons of Augustus, was afterwards customarily conferred upon the probable successor to the throne on his first entry into public life. (On other uses of princeps in antiquity, especially as applied to Augustus and his successors, see princeps n. and adj.)

Here are the definitions for sense 6 and sense 7 in OED for reference:

6. The ruler of a principality or small state actually, nominally, or originally subject to a king or emperor.
7. A male member of a royal family other than a reigning king (†in early use also a princess); esp. in the United Kingdom, a son or grandson of a monarch (also as a prefixed title).

Answer 2

I need to make one small correction to your question. There are several misconceptions about Latin and Roman rule. One is that One is that the Latin 'imperium' means empire and the other is that the Roman emperor was called 'imperator'. The title 'imperator' was honorific and meant that he had been hailed 'imperator' by his troops at the end of some great victory. 'imperium' was the Latin for the sphere of command allocated to a senator after his year as consul, or quaestor. The term 'imperium' meant 'sphere of command'. So, after his year as consul, as ex-consul would be given a provincia, and his imperium to raise troops as necessary, levy taxes, keep the peace held exclusively within the boundaries of the provincia. Inside it, he had the imperium (power), but outside it, he had no power at all. He was just an ordinary citizen. We owe the use of the word 'imperalistic' word imperium to the Roman poet Virgil who has Jupiter promising that Rome (meaning his Roman readers to think, of course, of the then current emperor, Augustus Caesar) was destined to have fame and 'imperium sine fine' - unbounded imperium. Each individual province of each provincial governor had a defined finis (boundary). Augustus' authority had none.

You are right, though, that the Roman 'emperor' had the title 'princeps' and that this reflected the fact that the he had the permanent title of princeps senatus (head of the Senate). Roman historians of the first century CE, like Tacitus, regularly referred to the 'emperor' as princeps. But the princeps was never hereditary, even when it was temporarily dynastic. The emperor of the day decided who should follow after his death and at some stage gave him and associate in his powers, so that, when the emperor died, the successor was already in power. So there was never any need for a special title for 'heir apparent'.

The nearest Romance language to ancient Latin is Italian. And Italy by the early middle ages had fragmented back into a number of city states, not necessarily with any strict constitutional arrangements. The Italian word principe actually referred in Sicily to its ruler. The ruler of Venice was a Doge. But the word for king that prevailed in Italian became the Italian word re for the original Latin word for that role, rex.

William I gave his previous title Duke of Normandy to his William Rufus, his son and heir to the throne of England, later to be known as William II. The practice of calling all male offspring of the king Prince began, according to Etymonline, in the sixteenth century.

Answer 3

This does not directly answer the question, but it might help lead to it.

Source: Britannica

The title princeps originated under the Roman Republic, when it was held by the leading member of the Senate (princeps senatus). Thus, Augustus’ use of the title lent plausibility to his claim to be the restorer of republican institutions vitiated during the civil wars of the 1st century BC. In fact, he had replaced the oligarchy of the republic with his own autocratic rule. Under his successors principatus soon did come to mean autocracy. This usage gave rise to the medieval title “prince.”

I'm not well-versed in politics more so in Roman history, but reading the article mentioned below, the gist is that the title princeps does not give the holder the absolute authority. Instead, the princeps stands by the side of the consuls and is the head over all other magistrates. He does have a province of his own but regarding if he leads a country is obscure, technically. (Roman History)

The new form thus given to the principate it retained as long as it lasted: for the future the position of princeps is only occasionally and accidentally connected with the tenure of what continued to be in theory the chief magistracy of the state; and the princeps is, strictly speaking, not a magistrate at all; he stands by the side of the consuls and over the heads of all other magistrates, with a definite province of his own, but vested also with a pre-eminent authority in all departments of state

PS

You can refer to A Dictionary of Greek and Roman Antiquities (1890) for further reading about the title princeps, its history, and what authoritative power it grants.

Answer 4

The question was when Prince was first used as the predicate for the child or children of a King.

It is a difficult question to answer because there is no extant primary evidence to show when it was first used in Europe. Certainly the King of a European state was known as 'noble Prince' in the 14th and 15th centuries, but 'majesty/majeste', as a predicate for a king was not necessarily used until the 16th century. Henry VIII and rulers before were called 'your grace'and other flowery language but not 'majesty'. What is certain is that the predicate of Prince was one of a less substantive one than King, the monarch of a smaller territory than a kingdom, and would thus be attached to the children of a Royal family to show that they were at the apex of nobility, but not THE apex. When this usage started is a moot point.