Source : Webster's Revised Unabridged Dictionary (1913)

Mileage \Mile"age\ (?; 48), n.
   1. An allowance for traveling expenses at a certain rate per
      mile.

   2. Aggregate length or distance in miles; esp., the sum of
      lengths of tracks or wires of a railroad company,
      telegraph company, etc. [Written also {milage}.]

   {Constructive mileage}, a mileage allowed for journeys
      supposed to be made, but not actually made. --Bartlett.