Power of attorney

Origin

From attorney ("any personal representative"). Attorney now rarely has that sense outside of fixed expressions like this one.

Full definition of power of attorney

Noun

1. (uncountable, legal) The legal authorisation of one person to act as the agent of another.He confirmed that he had power of attorney.
2. (countable, legal) A legal document allowing one person to act as the agent of another.Here you can apply for a power of attorney.