In commerce, a broker is a party that meiates between a buyer and a seller. A broker who also acts as a seller or as a buyer becomes a principal party to the deal. Distinguish agent: one who acts on behalf of a principal. A brokerage or a brokerage firm is a business that acts as a broker. A sales person working for a securities or commodity brokerage firm is popularly (but incorrectly) called a "broker." A broker in that context is, strictly speaking, an exchange member who actually executes the purchase or sale order in the pit, on the exchange, as a service to the client of the firm for which that salesman works.