Plain English Meaning
The book the manufacturer produces for a specific airplane that tells the pilot how to operate it safely -- how to start it, fly it, what its limits are, how it performs, and what to do if something goes wrong.
Full Definition
The manufacturer-produced document that contains the operating procedures, performance data, limitations, weight and balance information, systems descriptions, and emergency procedures specific to a particular aircraft make, model, and serial number. For aircraft certificated after March 1, 1979, the Pilot Operating Handbook typically also serves as the FAA-approved Airplane Flight Manual when it contains the required approved sections.