Final answer:
A foreign key is an attribute in a table that serves as a unique identifier in another table and is used to link the two tables.
Step-by-step explanation:
An attribute in a table that serves as a unique identifier in another table and is used to link the two tables is a foreign key. It is a column in one table that refers to the primary key of another table, establishing a relationship between the two tables. The foreign key helps maintain data integrity and enforce referential integrity constraints.