Referential integrity constraint in a relational database is specified with the help of a foreign key.

The referential integrity constraint is a rule that ensures the consistency and accuracy of data in a relational database. It specifies that values in one table's foreign key column must match the values in another table's primary key column. This constraint helps maintain the relationships between tables and prevents inconsistencies or errors in the data.

Foreign Key:
A foreign key is a column or a set of columns in a table that refers to the primary key of another table. It establishes a link between two tables based on a common field. The foreign key acts as a cross-reference between the tables and represents the relationship between them. In other words, it provides a way to connect data from multiple tables in a relational database.

Role of Foreign Key in Referential Integrity Constraint:
When a foreign key is defined in a table, it establishes a relationship between that table (known as the referencing table) and another table (known as the referenced table). The foreign key column in the referencing table contains values that match the primary key values in the referenced table. This relationship is enforced by the referential integrity constraint.

The referential integrity constraint ensures that the values in the foreign key column of the referencing table are always valid and consistent. It prevents the insertion of values that do not exist in the referenced table's primary key column. If an attempt is made to insert such invalid values, the database management system will raise an error and reject the operation.

Similarly, the referential integrity constraint also maintains the integrity of the data during updates and deletions. If a referenced primary key value is modified or deleted, the database management system can automatically update or delete the corresponding foreign key values in the referencing table to maintain consistency.

In summary, the foreign key plays a crucial role in specifying and enforcing the referential integrity constraint in a relational database. It ensures that the relationships between tables are maintained and that the data remains accurate and consistent.