What is referential_action?

Four times, the word "referential_action" is used on this page, but it's
never mentioned what the possible referential actions are.

Am I missing something?

https://www.postgresql.org/docs/14/sql-altertable.html

[ CONSTRAINT constraint_name ]
[snip]
PRIMARY KEY index_parameters |
REFERENCES reftable [ ( refcolumn ) ] [ MATCH FULL | MATCH PARTIAL |
MATCH SIMPLE ]
[ ON DELETE referential_action ] [ ON UPDATE referential_action ] }

[ CONSTRAINT constraint_name ]
[snip]
FOREIGN KEY ( column_name [, ... ] ) REFERENCES reftable [ ( refcolumn [,
... ] ) ]
[ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ] [ ON DELETE
referential_action ] [ ON UPDATE referential_action ] }