/ ˈdemənstreɪtə(r); NAmE / noun
1.
a person who takes part in a public meeting or march in order to protest against sb/sth or to show support for sb/sth
2.
a person whose job is to show or explain how sth works or is done
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WORD ORIGIN
early 17th cent. (in the general sense a person or thing that demonstrates or proves ): from Latin , from demonstrat- pointed out, from the verb demonstrare .