PNR stands for Passenger Name Record. Sometimes also called Booking Reference. PNR is used to record reservation and passenger information in airline and travel agency databases. The traveler's flight numbers, fare, mileage program numbers, etc. are stored within the PNR.
PNR can also mean the unique reference number for a booking, although the correct term in this case is 'Record Locator'. A Record locator is an alphanumeric reference number used by airlines and travel agencies to identify an individual booking. It usually consists of between 5 and 7 letters or numerals, thus allowing a sufficient number of unique combinations and negating the need to duplicate over time. Airline systems only work in upper case, but this does not unduly restrict the possible combinations.