A domain is a set of atomic values, has an a data-type / format defined for it and has a logical definition:
- Example: “USA_phone_numbers” are the set of 10 digit phone numbers valid in the U.S. They may have a format “(ddd)ddd-dddd” where each d is a decimal digit. The attribute name designates the role played by a domain in a relation, it is used to interpret the meaning of the data elements as they correspond to that attribute.