An illusion is a distortion of perception. The brain arranges, sorts, and organises data from the senses. Normally the system works well. Sometimes it does not, and we see illusions. In general, they are shared by most people in the same situation.
Illusions can happen with all five senses (taste, touch, sight, smell, and hearing), and some involve the way information from two senses is put together. Some illusions happen because of disorders, but generally, all normal people can sense the same illusion. An illusion is different from a hallucination; a halucination is sensing something which is not real, but an illusion is interpreting what we sense wrongly.