What is the legal document called that authorizes someone to act on behalf of another?

The legal document that authorizes someone to act on behalf of another is known as a Power of Attorney. This document grants specific powers to an individual, called the agent or attorney-in-fact, allowing them to make decisions regarding the principal’s financial, legal, or medical affairs. The versatility of a Power of Attorney is significant; it can be tailored to provide broad powers or limited to specific actions, depending on the principal's preferences.

A Will and Testament serves a different purpose, as it outlines the distribution of a person's assets after their death and does not delegate decision-making authority to another individual. A Contract Agreement, while a binding arrangement between parties, does not inherently assign authority to act on behalf of one party. An Affidavit is a written statement confirmed by oath, used primarily as evidence in legal proceedings, and does not confer the power to act on someone else's behalf. Thus, a Power of Attorney is the correct answer in this context.