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.
2. The mode in which the waters of a country pass off by its
streams and rivers.
3. (Engin.) The system of drains and their operation, by
which superfluous water is removed from towns, railway
beds, mines, and other works.
4. Area or district drained; as, the drainage of the Po, the
Thames, etc. --Latham.
5. (Surg.) The act, process, or means of drawing off the pus
or fluids from a wound, abscess, etc.
Drainage tube (Surg.), a tube introduced into a wound,
etc., to draw off the discharges.