agent

noun:a person authorized to act on behalf of another, especially in business or legal matters
- The lawyer served as the agent for the company's merger negotiations.
noun:a spy or covert operative who gathers information or carries out secret missions
- The novel follows a former CIA agent who uncovers a global conspiracy.
noun:a substance that causes a specific effect, especially a chemical that produces a physiological response
- The doctor prescribed a new anti‑inflammatory agent to relieve the patient's pain.