The notion of norms enters the broad field of Translation Studies with Toury's essay 'The nature and role of norms in Translation Studies', first published in In Search of a Theory of Translation (1980b) and reproduced in somewhat more than its entirety in his Descriptive Translation Studies and Beyond (1995), to which my page references will be. Here, norms are understood as sociocultural phenomena (Toury, 1995: 62) situated between two extremes of a scale of sociocultural constraint: Absolute rules at one end and complete idiosyncrasy at the other:

Rules N O R M S Idiosyncrasy

A norm may be more or less close to one of these extremes and its position on the scale is subject to change, disappearance and appearance over time (Toury, 1995: 54); that is, norms are basically unstable (p. 62).

Norms are assimilated by individuals in the course of their socialisation process, and adherence to them or deviance from them has the potential to incur approval or sanction of various kinds, including positive or negative criticism (p. 55). Norms are not directly observable, but they can be learnt and also studied through observation of patterned, recurrent behaviour, for example in talk aloud protocol studies, or through observation of the immediate results of translational behaviour, texts.

Translation scholars may consider these data in light of what is known about extratextual factors such as translation policy, publishing constraints, sociocultural mores and customs and so on (Toury, 1995: 65) to arrive at verbalisations of the norms; but outside of theory, norms are rarely explicitly verbalised and tend to be followed by translators almost unawares.

This understanding of the norm concept does not differ substantially from that evidenced in general, socioculturally oriented definitions of norms, such as the following:

Expectations of how a person or persons will behave in a given situation based on established protocols, rules of conduct or accepted social practices. 

Away of behaving or believing that is normal for a group or culture. All societies have their norms, they are simply what most people do. Deviants break norms. Some norms are enshrined in law and society punishes those who deviate from them. Breaches of unwritten norms are unofficially punished. This is important to science, because innovation is a form of deviancy science formally encourages. (Hewitt, 2005: Glossary)

An expected standard of behaviour and belief established and enforced by a group.

Shared belief that a person ought to behave in a certain way at a certain time. (Stafford & Scott, 1986: 81)

These definitions highlight both the social and the expectational nature of norms. Norms belong to social groups. They are not absolute (legislative) rules, and people are able to contravene norms as well as to adhere to them.

Although norms, in this sense, obviously relate to systems of belief among groups of people about what is appropriate behaviour at a certain time in certain circumstances, it is important to note that what people believe should be done may not necessarily be what even those who hold the belief actually do. In the social and socially applied sciences, it is customary, therefore, to distinguish between attitudinal norms, which have to do with 'shared beliefs or expectations in a social group about how people in general or members of the group ought to behave in various circumstances' (Perkins, 2002: 165), and behavioural norms, which have to do with 'the most common actions actually exhibited in a social group' (Perkins, 2002: 165). Attitudinal norms do not necessarily determine behaviour; as Perkins puts it (2002: 165): 'How most other community members believe everyone should behave and what behaviour is most common may be correlated, of course, but each component may also be somewhat distinct'. Clearly, a distinction might similarly be drawn in Translation Studies between what people's (including translators') attitudes are to translational phenomena, which might be partially tapped by protocol and interview studies, and what some people (translators) actually do when translating.