exponent
Pronunciation: /ɪkˈspəʊnənt, ɛk-/
noun
- 1a person who supports an idea or theory and tries to persuade people of its truth or benefits: an early exponent of the teachings of Thomas Aquinas
- a person who demonstrates a particular skill to a high standard: he’s the world’s leading exponent of country rock guitar

Origin:
late 16th century (as an adjective in the sense 'expounding'): from Latin exponent- 'putting out', from the verb exponere (see expound)