For relationships enforcing referential integrity between tables, cascading update is an option that causes a change to the primary key in a record in the primary table to automatically update the foreign key in all related records in the related foreign table or tables.
For example, if you establish a relationship between a Customers (primary) table and an Orders (foreign) table with the cascading update option enabled, when the primary key in a record in the Customers table is changed, all orders associated with that customer would also be changed in the foreign Orders table.
Now also definable from SQL DDL.