What is an illusion?
The Webster dictionary defines "illusion" as: “the perception of something objectively existing in such a way as to cause misinterpretation of its actual nature”
How do you perceive things? If through language, then you’re misinterpreting the nature of reality, because language is so scrawny. There’s absolutely no way language can capture the fullness of being.
If you look at things, sure—you can see them, but how well can you see something? Your eye is going to leave something out. You can get a microscope, but those have a limit, too.
If you touch things, then you can feel them only because your body has a special relationship to whatever you’re touching. You normally feel things only when your body is somehow denser than the material you’re touching (for example we don't "feel" air in the everyday sense).
How about hearing things? The result of hearing something is manufactured, and subject to the definition of "illusion". as well because humans have specific thresholds as to what they can hear. For example, a dog can hear higher pitched sounds than humans can.
We have only a perception of reality composed from these vignettes created by our senses and memories. And the perception is always going to be indirect, and subject to being "an illusion".