Allusion and illusion differ by only one vowel, but a single vowel is all it takes. The two words could not be more different from one another.
An allusion is an indirect reference. It’s a common technique in academics generally and literature specifically. Not a single college student has gone through her academic career without making an allusion to some sort of historical precedent or figure. If that college student is a student of literature, she has more likely than not made references to Shakespeare, Twain, or any other number of writers.
An illusion is a deceptive appearance. It’s the trademark of magicians. Card tricks? An illusion. Few people in the audience may know how the trick works, but they know it’s not the result of “magic.” The trick is an act of illusion, a sleight of hand, a willingness to believe in the unbelievable and improbable.
As to how the two words get to be confused with one another, the confusion may rest upon spellcheck and proofreading more so than a confusion about what the two words mean. The words, like so many other word pairs, simply require extra care when proofing.
Have a question for Write Right? Leave one in a comment or on the Facebook page.