Revenue passenger mile

Revenue passenger miles (RPMs) is a measure of a passenger traffic for an airline flight, bus, or train calculated by multiplying the total number of revenue-paying passengers aboard the vehicle by the distance traveled measured in miles. The calculation can be more complex for long-distance buses and trains (and some planes) where passengers may board and disembark at intermediate stops. Non-revenue-paying passengers (e.g., airline employees flying on free or nearly-free passes, passengers using free tickets, etc.) are excluded when measuring RPMs. Passengers who paid for their trip with a frequent-flyer program mileage award are usually included.

From http://en.wikipedia.org/