In sociology, a norm, or social norm, is a pattern of behavior expected within a particular society in a given situation. The shared belief of what is normal and acceptable shapes and enforces the actions of people in a society. Those who do not follow their social norms are considered eccentric or even deviant and are typically stigmatized. The very fact that others in one's society follow the norm may give them a reason to follow it. Important norms are called mores. Violations of mores are usually punished with severe sanctions, and are often enforced by law.
A norm may or may not have a rational justification or origin. Norms with common sense origins may, over time, lose their original context as society changes: an action that was once performed because it was necessary to survive may over the years become a social norm, even once the circumstances that made it necessary for survival are no longer applicable. There are at least two reasons for the stability of a norm. First, people are educated via thier socialization process to follow a norm and most people will not oppose it. Second, even if a person does not feel like following a norm, it may be in his best interest to follow it anyway.
Traditional norms such as the Golden rule have been followed by many people over a long period of time. Therefore norms are closely related to customs. On the other hand, a norm may arise as a formal description of an implicitly followed custom (see custom (law) for example).
In social situations, such as meetings, norms are unwritten and often unstated rules that govern individuals' behavior. Norms are most evident when they are not followed or are broken. This is often experienced when an individual finds him/herself in a foreign country, dealing with a strange culture where the norms are different. By the same token, import of cultural products in a culture may confront its people with different norms than they take for granted. Cultural import may then be seen as a threat to cultural identity.
In some groups, norms are consciously prescribed as a set of ground rules.
Persons skilled in facilitation assist groups in recognizing norms, as well as establishing norms to promote greater group (or team) effectiveness.
A general formal framework that can be used to represent the essential elements of the social situation surrounding a norm is the repeated game of game theory.
Example (gift exchange)
In the western world, it is a custom to exchange gifts in the holiday seasons. It is so deeply imprinted in the minds of people that many do not think of acting otherwise.
Now, suppose you become fed up with exchanging gifts, it is not necessarily easy to change your action. Unilaterally changing your action to stop giving gifts may give others the impression that you are a selfish person, and that impression may not be in your interest.
Notice however that the fact that your friends follow the custom may not necessarily imply that they are willing to do so. They may be following the norm for the reason exactly the same as yours. The situation resembles that in the short story of The Gift of the Magi. All the friends have to coordinate to change the custom.