Principal Definition: An agent’s master; the person for whom an agent has received instruction and to whose benefit the agent is expected to perform and make decisions. Related Terms: Agent, Ratification, Agency An agent’s master; the person for whom an agent has received instruction and to whose benefit the agent is expected to perform and make decisions. Further reading: see the extensive article on Agency Law. Find you are constantly looking up definitions? Try our search provider (works in most modern browsers) If you find an error or omission in Duhaime's Legal Dictionary, or if you have legal term suggestion, we'd love to hear from you!