An Illusionist is a Magic user that primarily uses his/her powers to trick others into sensing something that isn’t really there.
For example, many illusionists can create coordinated light and sound to make an apparition appear, such as cold fire or even a walking and talking person. The ability can be extended to touch with very precise uses of telekinesis. A more advanced technique would be using Mental or Psychic Magic to make an illusion directly in someone else’s mind.
There are advantages and disadvantages to these different methodologies. When trying to fool a big crowd, making the light is much easier than trying to use mental magic on everyone there, but for the most convincing illusion possible on one person using mental magic is likely the best way (unless they have been trained against such attacks).