A Proxy

1. An agent or substitute authorized to act for another person.
2. The authority to act for another, especially when written.
3. The written appointment of a proctor in suits in the ecclesiastical courts.
4. (sciences) A measurement of one physical quantity that is used as an indicator of the value of another
5. (software) An interface for a service, especially for one that is remote, resource-intensive, or otherwise difficult to use directly.

deputy

substitute

representative