Plain English Meaning
The official book from the aircraft's manufacturer that tells the pilot exactly how to operate that specific airplane -- how it performs, what its limits are, how its systems work, and what to do in an emergency.
Full Definition
A document produced by the aircraft manufacturer that contains the specific operating procedures, performance data, limitations, weight and balance information, systems descriptions, and emergency procedures for a particular make, model, and serial number of aircraft. For aircraft certified after March 1, 1979, the POH is typically also the FAA-approved Airplane Flight Manual (AFM), or contains the AFM as a section within it.