The Collaborative International Dictionary of English v.0.48:

Drainage \Drain"age\, n.
   1. A draining; a gradual flowing off of any liquid; also,
      that which flows out of a drain.
      [1913 Webster]

   2. The mode in which the waters of a country pass off by its
      streams and rivers.
      [1913 Webster]

   3. (Engin.) The system of drains and their operation, by
      which superfluous water is removed from towns, railway
      beds, mines, and other works.
      [1913 Webster]

   4. Area or district drained; as, the drainage of the Po, the
      Thames, etc. --Latham.
      [1913 Webster]

   5. (Surg.) The act, process, or means of drawing off the pus
      or fluids from a wound, abscess, etc.
      [1913 Webster]

   Drainage tube (Surg.), a tube introduced into a wound,
      etc., to draw off the discharges.
      [1913 Webster]

WordNet (r) 3.0 (2006):

drainage
    n 1: emptying something accomplished by allowing liquid to run
         out of it [syn: drain, drainage]

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "drainage":
   air-drying, anhydration, aspiration, bleeding, bloodletting,
   broaching, cleaning out, clearance, clearing, cupping, defecation,
   defluxion, dehumidification, dehydration, depletion, desiccation,
   discharge, discharging cargo, drafting, drain, draining, drawing,
   drying, drying up, effluence, efflux, effluxion, effusion, egress,
   elimination, emptying, evacuation, evaporation, excretion,
   exhalation, exhaust, exhausting, exhaustion, flowoff, gush,
   insolation, milking, mummification, off-loading, outfall, outflow,
   outflowing, outpour, outpouring, phlebotomy, pipetting, pumping,
   removal, runoff, siphoning, sucking, suction, tapping, unloading,
   venesection, venting, voidance, voiding, withering