exponent

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

• 2 Mathematics a quantity representing the power to which a given number or expression is to be raised, usually expressed as a raised symbol beside the number or expression (e.g. 3 in 2³ = 2 × 2 × 2).

• 3 Linguistics a linguistic unit that realizes another, more abstract unit.