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.*
*(Pope: The Rape of the Lock)
Here is an allusion to the dilemma of Aeneas, the hero of Virgil's Aeneid. Aeneas falls in love with Dido, the queen of Carthage. Dido implores Aencas to marry her and get settled pcrmanently in Carthage. Aeneas was in dilemma. He had a noble duty to find out a new territory for the Irojans. But he was also deeply in love with Dido. He was torn between love and duty. However, he finally decides to continue his voyage in search of a permanent empire for the Trojans. This dilemma of Aeneas has been recalled here to suggest the intensity of Belinda's crisis.
More examples:
The winged boy I knew;
But who wast thou, O happy, happy dove?*
*(Keats: "Ode to Psyche")
The winged boy" is an allusion to Cupid, the god of love.
perhaps the selfsame song that found a path
Through the selfsame song that found a path the sad heart of Ruth, when, sick for home
She stood in tears amid the alien corn ;*
*(Keats: "ode to a Nightingale")
These lines allude to the suffering of Ruth, a the old character of Testament.
Here is another example :
Imagine with thy self, courteous Reader, how often I then wished for the Tongue of Demosthenes or Cicero, that might have enabled me to celebrate the praise of my own de native Country in a Style equal to its Merits and Felicity *
*(Swift: Gulliver's Travels, Part Il)
The allusion in these lines is to the power of eloquence of Demosthenes and Cicero.
An allusion, which clarifies meanings and suggests a lot in a few words, may make a literary work difficult but it enriches its literary quality.