If you participate in a political protest, you are a demonstrator. Demonstrate means to show, and by dint of your presence, you are showing that you agree with the protest message.

Demonstrator can also mean someone who demonstrates how something works. Electrolux vacuums were sold by door-to-door salespeople who were also demonstrators––they put horrible messes down on the rug to demonstrate their vacuum's power.

Definitions of demonstrator

n someone who participates in a public display of group feeling

someone who demonstrates in opposition to another demonstration
a protester posted by a labor organization outside a place of work
Type of:
crusader, meliorist, reformer, reformist, social reformer
a disputant who advocates reform

n someone who demonstrates an article to a prospective buyer

sales demonstrator
Type of:
sales rep, sales representative, salesperson
a person employed to represent a business and to sell its merchandise (as to customers in a store or to customers who are visited)

n a teacher or teacher's assistant who demonstrates the principles that are being taught

Type of:
instructor, teacher
a person whose occupation is teaching

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