A passing reference to a literary or historical person, place, or event or to another literary work or passage is called-

Allusion:
An implicit or indirect reference to another work of literature, a historical or mythical person or event.
Example:
''Not half so fixed the Trojan could remain,
While Anna begged and Dido raged in vain'' is an example of allusion.
(Pope: The Rape of the Lock)

Source: An ABC of English Literature - Dr. M Mofizar Rahman.