Final answer:
The attribute with a unique value serving as the primary identifier in a database table is called a primary key. Composite keys aggregate multiple attributes to ensure uniqueness, while unique keys allow for singular, nullable distinctness. Foreign keys relate to primary keys in other tables for data integrity.
Step-by-step explanation:
An attribute for the entity set that has a unique value and is underlined, indicating its role as a primary identifier, is most commonly known as a primary key. In database management systems, a primary key is a field or combination of fields that uniquely identifies each record in a table, and no two rows can have the same primary key value. An entity set can also have a composite key, a unique key, or a foreign key, each serving different purposes. Composite keys are made up of two or more attributes that together ensure uniqueness, whereas unique keys are similar to primary keys but allow for one null value and can have multiple unique constraints in a single table. Foreign keys, on the other hand, are used to link two tables together by referring to the primary key of another table, ensuring referential integrity within the database.