Referential integrity is the automatic enforcement of referential constraints that exist between a reference table and a referencing table. When referential integrity is enforced, the value of a foreign key exists as a primary key value in the reference table. In other words, when referential integrity is enforced, all of the foreign key values in, for example, the department code column in an employee table exist as primary key values in a department table.
In other words, "Referential integrity" refers to the consistency that must be maintained between primary and foreign keys, i.e. every foreign key value must have a corresponding primary key value.