An impressionist or a mimic is a performer whose act consists of imitating sounds, voices and mannerisms of celebrities and cartoon characters. The word usually refers to a professional comedian/entertainer who specializes in such performances and has developed a wide repertoire of impressions, including adding to them, often to keep pace with current events. Impressionist performances are a classic casino entertainment genre.
Someone who imitates one particular person without claiming a wide range, such as a lookalike, is instead called an impersonator. In very broad contexts, "impersonator" may be substituted for "impressionist" where the distinction between the two is less important than avoiding confusion with the use of "impressionist" in painting and music.