Demonstrator

['demənstreɪtə] or ['dɛmən'stretɚ]

(noun.) someone who participates in a public display of group feeling.

(noun.) someone who demonstrates an article to a prospective buyer.

(noun.) a teacher or teacher's assistant who demonstrates the principles that are being taught.

Edited by Jeremy--From WordNet

(n.) One who demonstrates; one who proves anything with certainty, or establishes it by indubitable evidence.

(n.) A teacher of practical anatomy.

Typist: Ruben