An attribute is a property that describes a characteristic. Usually, attributes refer to single property instances, i.e. to property instances, which contain exactly one element. Attributes may, however, also refer to collections. In contrast to associations, the attribute value is the value of its instance value(s).
Attributes are specific properties, which are owned by the object instance. Usually, attributes are embedded in the object instance. Attributes are associated with a data type, which might be a basic or a complex data type.