A person appointed to act for another person (the donor).
A formal document called a Power of Attorney is used to appoint the attorney. They can then operate the bank account(s) of the person giving them the Power of Attorney.
People do not have to be legally qualified to be appointed as an attorney. Enduring Powers of Attorney, and since October 2007 Lasting Powers of Attorney; are used to authorise an attorney who can continue to act if the donor loses the capacity to look after their own affairs.