An allusion is a significant reference to a person, place, idea, event, or work. For example, calling a grumpy, especially greedy person a "Scrooge" is a literary allusion, because it references the character Scrooge from Charles Dickens' A Christmas Carol, who was a mean, greedy man until his time with the three spirits changed his heart. Calling a passerby who decides to help out a "good Samaritan" is a Biblical allusion that references a parable in the New Testament. While many allusions are made to works of literature, it can be to famous people, historical events, etc. The key is that they are significant. Sometimes this significance can be hard to spot because the allusions are made to things that are no longer culturally relevant or widely read, so it would take a particularly studious person to be able to pick up on them without a little bit of extra research.