What is the difference between a thing that is real and a thing that is an illusion? So, is Maya an illusion or is it real?
Notice that the word illusion can be used in more than one way. A magic trick is an illusion. What you're seeing isn't what you think you're seeing. So is a mirage of an oasis if it looks like a place to get a drink. So is a rainbow if it looks like something you could walk to or climb across. With a little help, you can see that none of them were what they appeared to be using the same senses that were previously deceived.
But the word is used in another sense, wherein it is claimed that nothing is as appears, a condition that cannot be corrected using the senses, as with your second comment regarding Maya, or calling duality illusion. One might learn that things aren't how they appear, but he can't experience them otherwise. Like rainbows, solid things like mountains aren't really solid, but the difference is that they can be climbed. Nothing is bright or red, but this illusion is different from a standard optical illusion, where we CAN be shown that two colors are actually the same despite being made to look different. Maybe you've seen this, which is the first kind of illusion, the kind that CAN be demonstrated as such to the senses:
An unfortunately, the two are usually conflated, as if the rainbow and mountain are alike, as if the mountain will ever behave like the rainbow and not behave as we expect a mountain to behave, as if a different understanding of a mountain should affect our approach to it.
