Nobility is a state-privileged status which is generally hereditary. Titles of nobility are usually associated with present or former monarchies. The term originally referred to those who were "known" or "notable" and was applied to the highest social class in pre-modern societies. In the feudal system (in Europe and elsewhere), the nobility were generally those who held a fief, often land or office, under vassalage, i.e., in exchange for allegiance and various, mainly military, services to the Monarch and at lower levels to another nobleman. It rapidly came to be seen as a hereditary caste, sometimes associated with a right to bear a hereditary title and, for example in pre-revolutionary France, enjoying fiscal and other privileges. While noble status formerly conferred significant privileges, today, in most Western countries, noble status is a purely honorary dignity. In the United Kingdom, where some peerage titles, until recently, guaranteed a seat in the Upper House of the Parliament, there are still some residual privileges.
Nobility is a historical, social and often legal notion, distinct from socio-economic status which is mainly based on income and possessions. Being wealthy or influential does not automatically make one a noble, nor are all nobles wealthy and influential (aristocratic families have lost their fortunes in various ways, and the concept of the 'poor nobleman' is almost as old as nobility itself). Countries without a feudal tradition do not have a nobility as such.
Various republics, including the United States, Mexico, and Italy have expressly abolished the granting and/or use of titles of nobility - although the latter two did so formerly. Although many societies have a privileged 'upper class' with great wealth and power, the status is not necessarily hereditary and does not entail a separate legal status, or different forms of address.
European nobility originated from the feudal/seignorial system that arose in Europe during the Middle Ages. Originally, knights or nobles were mounted warriors who swore allegiance to their sovereign and promised to fight for him in exchange for an allocation of land (usually together with serfs living there). During the time period known as the Military Revolution, nobles gradually lost their role of raising and commanding private armies, as many nations created cohesive national armies. This was coupled with a loss of the social-economic power of the nobility, owing to the economic changes of the Renaissance and the growing economic importance of the merchant classes (or bourgeoisie), which increased still further during the Industrial Revolution. In countries where the nobility was the dominant class, the bourgeoisie gradually grew in power; a rich city merchant was more influential than a minor rural nobleman. However, in many countries at this time, the nobility retained great social and political importance; for instance, the UK's government was dominated by the nobility until the middle of the 19th century and thereafter the powers of the nobility were progressively reduced by legislation (see Reform of the House of Lords).
The nobility of a person might be either inherited or earned. Nobility in its most general and strict sense is an acknowledged preeminence that is hereditary: i.e., legitimate descendants (or all male descendants, in some societies) of nobles are nobles, unless explicitly stripped of the privilege. In this respect, nobility is distinguished from the peerage: the latter can be passed to only a single member of the family. The terms aristocrat and aristocracy are a less formal means to refer to persons belonging to this social milieu. Those lacking a distinct title, such as junior siblings of peers (and perhaps even the children of 'self-made' VIPs) may be considered aristocrats, moving within a small social circle at the apex of a hierarchical social pyramid.
In France, influential high bourgeois, most particularly the members of the parlements (courts of justice), obtained noble titles from the king. The old nobility of military origin, the noblesse d'épée ("nobility of the sword") became increasingly irritated by this newer noblesse de robe ("nobility of the gown"). In the last years of the ancien régime, before the French Revolution, the old nobility, intent on keeping its privileges, had pushed for restrictions of certain offices and orders of chivalry to noblemen who could demonstrate that their family had enough "noble quarterings" (in French, 'quartiers de noblesse'), a reference to a noble's ability to display armorially their descents from armigerous noble forebears in each of their lines of descent to demonstrate that they were descended from old noble families, who bore arms that could be quartered with their own male line arms, and thus prove that they did not derive merely from bourgeois families recently elevated to noble rank (although historians such as William Doyle have disputed this so-called 'Aristocratic Reaction'. (W. Doyle, Essays on Eighteenth Century France, London, 1995). A noble could be asked to provide proof of noble antecedents by showing a genealogy displaying seize quartiers (sixteen quarterings) or even trente-deux quartiers (thirty-two quartering) indicating noble descent on all bloodlines back five generations (to great-great grandparents) or six generations (great-great-great grandparents), respectively. This illustrates the traditional link in many countries between heraldry and nobility; in those countries where heraldry is used, nobles have almost always been armigerous, and have used heraldry to demonstrate their ancestry and family history. However, it is important to note that heraldry has never been restricted to the noble classes in most countries, and being armigerous does not necessarily demonstrate nobility. A noted opinion of Innes of Learney, makes an observation of the system in use in Scotland, and notes the differences there from many other European traditions given that all legal armorial bearings, which are entered in the Public Register of All Arms and Bearings in Scotland, by warrant of the Lord Lyon King of Arms, are by statute "Ensigns of Nobility". However, this opinion is challenged by scholars 
Nobles typically commanded resources, such as food, money, or labor, from common members or nobles of lower rank of their societies, and could exercise religious or political power over them. Also, typically, but not necessarily, nobles were entitled to land property, which was often reflected in the title. For example, the title Earl of Chesterfield tells about property, while the title Earl Cairns was created for a surname. However all the above is not universal; quite often nobility was associated only with social respect and certain social privileges. An example of the latter would be early 20th-century Polish nobility (szlachta) after their political, economic, judicial and religious privileges were abolished in 1921 and they remained only landed proprietors on the same legal basis as their landed-commoner neighbours. In the modern age, the notion of inherited nobility with special rights has become, in the Western World, increasingly seen as irrelevant to the modern way of life. The founding fathers of the United States rejected anything that could have helped in recreating a nobility; the French Revolution abolished the nobility and its special privileges (though some nobility titles would be recreated by Napoleon I and III, they were mostly honorific).
Some con artists also sell fake titles of nobility, often with impressive-looking documents to back them up. These may be illegal, depending on local law. They are more often illegal in countries that actually have nobilities:such as European monarchies. In the U.S., such commerce would be a form of fraud, but it would only victimize the buyer of the supposed titles and would not threaten an established class of nobles with enforceable titles.
Blue blood is an English idiom recorded since 1834 for noble birth or descent; it is a translation of the Spanish phrase sangre azul, which described the Spanish royal family and other high nobility who claimed to be of Visigothic descent, in contrast to the Moors. It is likely that the idiom originates from ancient and medieval societies of Europe and distinguishes an upper class (whose superficial veins appeared blue through their untanned skin) from a working class of the time; mainly agricultural peasants who spent most of their time working outdoors and thus had tanned skin (through which superficial veins appear less prominently). An alternative traditional explanation, argyria (a disease causing a blue-grey skin tone after digestion of silver), is considered less valid, as table silverware was not yet regularly used by much of the nobility. Another explanation has also been put forward: methemoglobinemia, caused by in-breeding.
Robert Lacey explains the genesis of the blue blood concept:
It was the Spaniards who gave the world the notion that an aristocrat's blood is not red but blue. The Spanish nobility started taking shape around the ninth century in classic military fashion, occupying land as warriors on horseback. They were to continue the process for more than five hundred years, clawing back sections of the peninsula from its Moorish occupiers, and a nobleman demonstrated his pedigree by holding up his sword arm to display the filigree of blue-blooded veins beneath his pale skin—proof that his birth had not been contaminated by the dark-skinned enemy. (Robert Lacey, Aristocrats. Little, Brown and Company, 1983, p. 67)
Medieval Japan developed a feudal system similar to the European system, where land was held in exchange for military service. The daimyo class, or hereditary landowning nobles, had great social and political power. Like their European counterparts, they commanded private armies made up of samurai, an elite warrior class; for long periods, these held real power without a real central government and often plunged the country into a state of civil war. Although there are differences, the daimyo class can be compared to European peers, and the samurai to European knights, but with important differences, such as the distinction between the European code of chivalry and the Japanese code of bushido. These feudal titles and ranks were abolished in Japan with the Meiji Restoration of 1868 and replaced by the kazoku, a five-rank peerage system after the British example which granted seats in the upper house of the Imperial Diet, but this too was abolished in 1947, following Japan's defeat in World War II.
Many other non-Western nations have had noble or aristocratic classes of various kinds: these are so diverse that it is somewhat misleading to try to translate them all into western feudal terminology. For the feudal hierarchy on the Indian subcontinent, see princely state.
In some Islamic countries, there are no definite nobility titles, but the closest to that are given the title Syed or Sayyid. This exclusive title, given only to certain descendants, literally means, 'Sir' or 'Lord'. There are no special rights concerning the title: they are considered more religious than the general population, and many people come to them for first-hand religious questions.
In Iran, the nobility titles are Mirza, Khan, ed-Dowleh, Shahzada, and so forth. These titles do not exist in the present day. An aristocratic family is now recognized by family name (often derived from the post held by their ancestors, considering the fact that family names in Iran only appeared in the beginning of the 20th century).
In East Asia the system was often modelled on imperial China, the leading culture, where the emperor conferred degrees of nobility, which were not permanent but decreased a rank each generation. Descendants of the Emperor formed the highest class of Ancient Chinese nobility, status based on ranks of the Empress or concubine (as the Emperor was polygamous), and numerous titles such as Taizi (Prince, Princess) were designated. Due to the complex shifts in dynastic rules, a succession of rules was introduced.
Dynasties established by the minority non-Han rulers via violent conquest in the later years disrupted this ancient system of social class within Han society, to conform to a racist and ethnic policy, where variously, the Mongols and the Manchus were accorded higher "genteel" status over the Han majority that they controlled.
By the Qing dynasty, many titles have been corrupted through abuse and perversion of the origin Qin system. Titles of nobility were still granted by the emperor, but served merely as honorifics based on a loose system of favors to the Qing emperor and Manchu interests: under a centralized system, governance in the empire was the responsibility of the Confucian-educated scholar-officials and local gentry. The literati were accorded gentry status based on lineage and for male citizens, advancement in status was possible via success in the top three positions in imperial examinations.
The establishment of titles was abolished with the establishment of the People's Republic of China in 1949 as part of a larger effort to abolish feudal influences on Chinese society.
In tribal societies, such as and the Polynesian Island states, the system of often (semi-)hereditary tribal chiefs can also be compared to a form of noble class; in Tonga, after Tongan contact with Western nations, the traditional system of chiefs developed into a Western-style monarchy with a hereditary class of barons, even adopting that English title.