The implicit (not spoken of…you learn about them through violation of the “rule”) or explicit (openly talked about) rules of a group concerning the appropriateness or inappropriateness of certain values, beliefs, attitudes, and behaviors.
It should be noted that these “rules” are not legal laws, instead they are more like social obligations. Most people follow the norms because the “group” may choose to punish the transgressor. Punishments could include: guilt/shame, talking behind their back/gossiping about the person, physical harm or even being made to leave the group permenantly.
Example: Although handshakes between men and women are common in most Western countries, in certain parts of the world such a gesture would be considered inappropriate contact between the sexes.
Fournier, G. (2010). Social Norms. Psych Central. Retrieved on September 5, 2015, from http://psychcentral.com/encyclopedia/2009/social-norms/