What are some examples of 'allusion'?

In literature, allusions refer to an outside event or source such as history, literature, religion, etc. What this means is that it pulls this information out of its original context and uses it to support the work. For example, a simile or metaphor for the "Garden of Eden" is an example of a biblical allusion. Any mention of Julius Caesar's death, or Brutus' betrayal, no matter how faint, is an allusion to history. Mentioning Shakespeare or Romeo and Juliet in another literary work is a literary allusion. Allusions are many.