An allusion is an indirect reference to a person, event or piece of literature. Allusion is used to explain or clarify a complex problem. Note that allusion works best if you keep it short and refer to something the reader / audience is familiar with (e.g. famous people,(Greek) mythology, the bible etc.). The advantages are as follows: we don't need lengthy explanations to clarify the problem; the reader becomes active by reflecting on the analogy; the message will stick in the reader's mind.

Example:

The software included a Trojan Horse. (allusion on the Trojan horse from Greek mythology)

Many allusions on historic events, mythology or the bible have become famous idioms.

Example:

to meet one's Waterloo (allusion on Napoleons defeat in the Battle of Waterloo).