From The Art and Popular Culture Encyclopedia
Nobility is a government-privileged title which may be either hereditary (see hereditary titles) or for a lifetime. Titles of nobility exist today in many countries although it is 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 and/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. Today, in most countries, "noble status" is a purely honorary dignity that confers no legal privileges; an important exception is the United Kingdom, where certain titles (titles of the peerage, until recently guaranteeing a seat in the Upper House of the UK Parliament, hence its name House of Lords), still confer some residual privileges.
Nobility is a historical, social and often legal notion, which should not be confused with socio-economic status which is mainly statistical 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 and Italy have expressly abolished titles of nobility. Although many such societies have a privileged 'upper class' with great wealth and power, this does not entail a separate legal status, or different forms of address.