demonstrator

Concise Oxford English Dictionary © 2008 Oxford University Press:
demonstrate/ˈdemənstreɪt/

▶verb

• 1 clearly show the existence or truth of.

  ■ give a practical exhibition and explanation of.

• 2 take part in a public demonstration.

– derivatives
demonstrator noun.

– origin C16: from L. demonstrat-, demonstrare ‘point out’.