The chair I'm sitting in isn't an illusion for any practical meaning of the word illusion.
Its solidity of the chair is, to a certain extent, illusory inasmuch as the odds of measuring a particle in any particular point in it at any given time are approximately zero, but my classical experience of the chair quite similar to any other human's experience of the chair, so the fact that there is an object there, it can be approximated by the word "chair" is pretty objectively true.
An illusion is a sensory stimulus that appears one way until you examine it closely, at which point it reveals itself to be other than what it initially appeared to be. Your chair absolutely fits that description.
Its solidity of the chair is, to a certain extent, illusory inasmuch as the odds of measuring a particle in any particular point in it at any given time are approximately zero, but my classical experience of the chair quite similar to any other human's experience of the chair, so the fact that there is an object there, it can be approximated by the word "chair" is pretty objectively true.