As I mentioned in an earlier Grammar 101 entry on the differences between allusion, illusion, and elusion, an allusion is when you make a reference to a book, movie, situation, person, or any number of other things. This is used extensively in both works of fiction and works of nonfiction, adding depth and context to the subject matter at hand.
As a general rule of thumb, when you allude to something, you are referring to it in an indirect way. You are suggesting it without explicitly mentioning it. If you are writing about a journey that someone is taking, you may allude to the “yellow brick road” from The Wizard of Oz. If you do not explicitly mention that movie, then you are making an indirect reference to it. Here’s an example:
“Jerry may have a long way to go before he can get home, but at least he has some companions with him on this yellow brick road.”
Jerry isn’t literally traveling on a yellow brick road; this is simply an indirect reference to the popular movie. Contrast the above sentence to the one below:
“Jerry may have a long way to go before he can get home, but just like Dorothy in The Wizard of Oz, he has companions. It’s almost like he is traveling a similar yellow brick road.”
Realistically, that’s not really an allusion anymore. You aren’t alluding to The Wizard of Oz; you’re referring to it. This connection is much more direct and unambiguous, as you are clearly spelling out that you are talking about The Wizard of Oz. It’s not just implied.
The next time you want to say that someone alluded to something, consider whether it is more appropriate to say that the someone referred to that something instead.