Agent (Agt.)

A person authorized to transact business for and in the name of another person or company. Types of agent are:
(1) brokers,
(2) commission merchants,
(3) resident buyers,
(4) sales agents,
5) manufacturer's representatives.

Source: U.S. Maritime Administration.

Someone that has authority to act for another.

Source: California Courts.

In immigrant visa processing the applicant selects a person who receives all correspondence regarding the case and pays the immigrant visa application processing fee.  The agent can be the applicant, the petitioner or another person selected by the applicant.

Source: U.S. Department of State.