Definition of Attribute
An attribute is a characteristic of an entity. Every entity has at least one attribute: its primary key (More accurately, a candidate key). Attributes are expressed as nouns qualified by adjectives that clarify their role.
An attribute plays one of three possible roles in any table:
Primary key attributes are said to be identifier attributes because they uniquely identify an instance of an entity.
A foreign key attribute can be an identifier attribute if it is part of a composite primary key; otherwise, foreign key attributes are descriptor attributes.
Nonkey attributes are said to be descriptor attributes because they specify a nonunique characteristic of an instance of an entity.
In the relational model, attributes have the same properties as they do in E-R theory.