What is an illusion?

Most define an “illusion” as where perception doesn’t match reality. Nah. That’s not what we really mean by it most of the time.

Basic point is, an illusion in the sense most of us really mean it is NOT simply "having a perception that differs from reality."

Instead, it's something more like, "the perception I'm having isn't even one of the perceptions one would expect from the stimulus image I'm receiving!"