Final answer:
In a relational database, when a primary key from one table appears in a different table, it is known as a foreign key. This key establishes a relationship between the two tables and ensures referential integrity in the database design.
Step-by-step explanation:
In a relational model, when a primary key for one table appears in another table, it is called a foreign key. A foreign key is a field (or collection of fields) in one table that uniquely identifies a row of another table or the same table. In essence, it creates a link between the two tables.
This is a fundamental concept in database design, ensuring referential integrity between data in different tables. A foreign key references the primary key of another table, creating a relationship between the two tables and allowing the relational database to be interconnected.