According to the Random House Dictionary, an allusion is "a passing or casual reference; an incidental mention of something, either directly or by implication." It is a common practice in literature to make allusions to a variety of sources. The following lists describe some common allusions to Greek or Roman mythology, the Bible, other works of literature, and history. These resources will help you determine why a character is described as having an "Achilles heel," the "patience of Job," or an "Oedipus complex." But hopefully not all three!
See also: Brewer's Dictionary of Phrase and Fable