What Is an Allusion?
An allusion is a reference, from a literary work to another work of fiction, a film, a piece of art, or even a real event. An allusion serves as a kind of shorthand, drawing on this outside work to provide greater context or meaning to the situation being written about. While allusions can be an economical way of communicating with the reader, they risk alienating readers who do not recognize these references.
Strong fiction (or poetry for that matter) will use allusions so that the fiction works on both levels. Readers who understand the allusions gain a richer understanding of the work, while those who don't can still follow the story and be entertained or enlightened by it.
Allusions are often thought of as a kind of hypertext, linking the reader to another tradition or literary history. Some work, such as T.S. Eliot's poem "The Wasteland" practically samples other works, much the same way DJs sample other songs. However, allusions can also be quite subtle. For instance, Shakespeare's influence on literature in English is so strong that allusions to his plays are often made without people being aware of it when they say, "don't act like a Romeo."
The Benefit of Using Allusions
Writers are often hard-pressed to come up with a descriptive way of getting across a point in a story. This is where allusions can be very helpful. For instance, imagine as a writer that you need to explain your main character's struggle against an overwhelming opponent. You want to get across the idea that the character is righteous and stands a chance of winning the battle, even though that chance appears to be a remote one. You could easily reference the confrontation as a meeting of "David meets Goliath." You are alluding to the well-known biblical story, the one of David and Goliath, to bring your reader's mind to the idea that the confrontation will like a one-sided battle but that the underdog stands a chance of triumph.
It's not a cop-out to use an allusion. It's an economy of words and it will move your story along quicker. One example of an obvious allusion is the phrase "that guy looks like a regular Adonis." This is a reference to the mythical figure of beauty Adonis. While the word is ancient, the reference (or allusion) is not. Another example is the phrase, "I feel like I am carrying the weight of the world on my shoulders." Again, you would be alluding to an ancient figure (that of Atlas who was portrayed holding the globe of the world on his shoulders) as a way to convey to your readers that your character feels burdened down.
Sometimes allusions can be hard to spot and these should be used sparingly. You don't want readers to have to constantly be running to a dictionary to look up a context. However, it may be fitting (especially if your work is a period piece) to use a less than obvious allusion. One example is Herman Melville who (in "Moby Dick") creates a sense of impending doom when he names the main ship the Pequod. Readers of Melville's classic may be aware that the Pequot people, a Native American tribe who were driven to extinction. The ship's name served to create a feeling of imminent destruction through the use of this allusion.