The Collaborative International Dictionary
Passenger mile
Passenger mile \Pas"sen*ger mile\ (Railroads) A unit of measurement of the passenger transportation performed by a railroad during a given period, usually a year, the total of which consists of the sum of the miles traversed by all the passengers on the road in the period in question.
Wiktionary
passenger mile
n. A basic unit used by transport companies for calculating profit levels etc. It is calculated by dividing total distance travelled in a given period by the number of passengers.