Jonkheer (female equivalent: Jonkvrouw) is a Dutch honorific of nobility. In Belgium, the title of Écuyer (in French) or Jonkheer/Jonkvrouw (in Dutch) is the lowest title within the nobility system, recognised by the Court of Cassation. It is the cognate and equivalent of the German noble honorific Junker, which was historically used throughout the German-speaking part of Europe, and to some extent also within Scandinavia.
Honorific of nobility
Jonkheer or Jonkvrouw is literally translated as "young lord" or "young lady". In medieval times such a person was a young and unmarried son or daughter of a high-ranking knight or nobleman. Many noble families could not support all their sons to become a knight because of the expensive equipment. So the eldest son of a knight was a young lord while his brothers remained as esquires.
However, in the Low Countries (and other parts of continental Europe), only the head of most noble families did and does carry a title, with inheritance via male lineage. This resulted therefore that most of the nobility was, and is nowadays, untitled in the Netherlands. 'Jonkheer', or its female equivalent 'jonkvrouw' developed therefore quite early into a different but general meaning, i.e., an honorific to show that someone does belong to the nobility, but does not possess a title. The abbreviation jhr., or jkvr. for women, is placed in front of the name (preceding academic, but not state titles).
The honorific could be compared more or less with "Edler" in Austria or "Junker" in Germany, though due to circumstances of German and especially Prussian history, "Junker" assumed connotations of militarism absent from the Dutch equivalent. Comparing it with the English nobility, it could be roughly translated as "The Honourable", when the untitled person is a son or daughter of a hereditary knight, baron, viscount or count, or "Lord" or "Lady", when the untitled person is member of the old (Dutch) nobility, untitled but of high ranking, and from preceding 1815 (i.e. "Heer van X" or Lord of X).
The spouse of a jonkheer is not named Jonkvrouw but is named "Mevrouw", translated into English as Madam, and abbreviated as "Mrs." (with the use of her husband's name). However, if she is a jonkvrouw in her own right, she can be styled as such (together with her maiden name), unless she chooses to use the name of her husband.
Title of nobility
Often however a title of nobility may be claimed by a family whose members are officially recognised only as jonkheeren, the title not being acknowledged by the modern monarchy either because the family is registered as untitled nobility and may thus only use the honorific or predicate, or because the family has not requested official registration of the title, but possesses a grant of nobility which predates the founding of the Kingdom of the Netherlands in 1815.
The coronet of rank for untitled nobility in the Netherlands and Belgium is the same as that of a Ridder, i.e. a hereditary knight: a plain circlet of gold with eight pearls, five of which are seen in a representation, all on golden points. Furthermore, on the heraldic coronet the golden circlet is surrounded with a pearl collar.
Unrecognised titleholders use the same coronet of rank as hereditary knights, described above. Unrecognised titles can not officially use a coronet of rank and thus use the coronet that they have been historically awarded, if any at all.
Jonkheer's most well-known use among English-speaking people is as the root of the name of the city of Yonkers, New York. The word was likely a nickname, as opposed to an honorific, associated with Adriaen van der Donck; a young Dutch law-maker, pioneering politician and landowner in New Netherland. While his business ventures largely proved less than successful, the city of Yonkers takes its name from his steadfast work in the formation of the state of Manhattan itself.
The word, in reference to Van der Donck, is variously spelled among modern scholars. In Thomas F. O’Donnell’s introduction to a translation of van der Donck’s A Description of the New Netherland, it is suggested that van der Donck was known as “The Joncker”. Russell Shorto’s The Island at the Center of the World has “jonker”, while Edward Hagaman Hall’s book on Philipse Manor Hall uses “youncker”. “Jonker” (old Dutch spelling joncker) is another form of the word “jonkheer”.