definition 

A legal document giving one person, called an agent or attorney in fact the power to act for another person, known as the principal.   A special or limited power of attorney gives a specific power to act in a narrowly defined situation or for specific assets.  A general power of attorney gives broad authorizations to the agent.  The agent may be able to make medical decisions, legal choices, financial and business decisions.

commentary

Most estate plan documents such as wills and trusts continue to be valid and useful for many years, unless there are family changes such as births, deaths, divorces, or changes in wishes.  However, Powers of Attorney, while technically valid for many years,  are often not accepted by financial institutions when older than a set period of years, which varies by institution.   It is good practice to have Powers of Attorney updated every five years.