An allusion is an expression or phrase that is meant to reference an event, person, place, thing, or idea without mentioning it explicitly. Typically, the things being alluded to are historically or culturally significant in some way. It is important to remember that an allusion is an indirect reference and it is left up to the audience to successfully make the connection.
“A mote it is to trouble the mind’s eye.
In the most high and palmy state of Rome,
A little ere the mightiest Julius fell,
The graves stood tenantless and the sheeted dead
Did squeak and gibber in the Roman streets”
This excerpt from Shakespeare’s Hamlet alludes to the assassination of Julius Caesar.
“Five score years ago, a great American, in whose symbolic shadow we stand today, signed the Emancipation Proclamation.”
Martin Luther King alludes to Abraham Lincoln by using the phrase “five score years” to call to mind the iconic opening of the Gettysburg Address ("Four score and seven years ago") and by mentioning the Emancipation Proclamation (which Lincoln signed).