What is the definition of a foreign key in a database?

A foreign key is defined as a reference to a primary key in another table. This relationship establishes a connection between two tables in a relational database, enabling the enforcement of referential integrity. When a foreign key is used, it ensures that the values in one table correspond to valid entries in another, creating a logical association between the data.

For example, in a database with two tables, "Customers" and "Orders," the "Orders" table may have a foreign key that references the "CustomerID" primary key in the "Customers" table. This setup allows one to quickly identify which customer placed a particular order, facilitating data retrieval and ensuring that every order is linked to an existing customer record.

The other options represent different concepts in database management. A unique identifier for a record refers to a primary key, which distinctly identifies each entry in its table rather than referencing another table. A collection of non-related fields does not define any relational context and does not describe a foreign key's purpose. Lastly, a type of index relates to performance optimization and does not inherently define the concept of foreign keys.