A synthespian is a synthetic actor or actress. Although they may be characters in a play or musical, they are not portrayed by an actual person.
For example, if Audrey II was simply a puppet(with no actor inside him), then Audrey II would be considered a synthespian. Another example of a synthespian would be if a production of some kind had a hand mirror. The mirror would not move, but would talk and be considered a character.