A reference in a literary work to a person, place, thing, or idea from history or pop culture; or to a work of art, music, or literature
In the following example from Romeo and Juliet by William Shakespeare, there are two allusions to Roman mythology: to Cupid, god of love; and Diana, goddess of chastity.
Well, in that hit you miss: she’ll not be hit
With Cupid’s arrow, she hath Dian’s wit (I.i.202-203)
The “Glossary of Literary Terms” website for UNC at Pembroke offers the following comment: Allusions are often used to summarize broad, complex ideas or emotions in one quick, powerful image. Romeo’s lines above express that his love interest, Rosaline, is not merely playing hard to get; she is uninterested in any romantic relationship. By saying she is impervious to Cupid’s arrow and shares a mindset with Diana, Romeo at once describes Rosaline’s remoteness and his deep frustration with it.
 http://www.uncp.edu/home/canada/work/allam/general/glossary.htm. Accessed January 7, 2013.