Plain English Meaning
When the aircraft ends up in a position the pilot didn't intend -- pointing too far up, too far down, or banked too steeply -- unusual attitude recovery is the standard procedure for getting it safely back to level flight.
Full Definition
The set of corrective control inputs a pilot makes to return an aircraft to straight-and-level flight after it has entered a pitch, bank, or airspeed condition well outside that intended for normal flight. In instrument flying, recovery is performed by to the (or, if it has failed, by reference to the , , , and ). The two standard cases are the recovery (add , lower the nose, the ) and the recovery (reduce power, level the wings, then raise the nose).