The term Nobility in this context refers to the nobles of Temma and the nobles of the world in general.
The nobility are a social class, with more privliges and eminence than other social classes. Membership in the nobility class is mostly hereditery: the children of nobles are nobles. However, in some cases, such as bastardom (both birth to two unmmaried people and birth to two people not married to each other), heinous crimes, or family disputes, membership in the nobility can be lost. In other cases, mostly adoption and marriage, people can enter the nobility class; but that is rare.
Non nobles are usually referred to as commoners, though that term is less used today since there are distinct classes within the non noble classes: guildmembers are civilians , like the nobility, and so are considered of a higher class than freemen, who are, again, of a higher class than serfs and slaves. In many cases, commoners would be used to mean freemen and slaves only, and not guildmembers.
The final authority on membership in the nobility class is the Court of Chivalry.