Power of Attorney
The American Bar Association...
...says that a power of attorney gives one or more persons the power to act on your behalf as your agent. Though accepted in all states, the rules and requirements differ. Power may be limited to a particular activity, or be general in its application. The power may give temporary or permanent authority to act on your behalf.
The power may take effect immediately, or only upon the occurrence of a future event, usually a determination that you are unable to act for yourself due to mental or physical disability. The latter is called a "springing" power of attorney. A power of attorney may be revoked, but most states require written notice of revocation to the person named to act for you.