The Collaborative International Dictionary of English v.0.48:

Evacuation \E*vac`u*a"tion\, n. [L. evacuatio: cf. F.
   ['e]vacuation.]
   1. The act of emptying, clearing of the contents, or
      discharging. Specifically:
      (a) (Mil.) Withdrawal of troops from a town, fortress,
          etc.
      (b) (Med.) Voidance of any matter by the natural passages
          of the body or by an artificial opening; defecation;
          also, a diminution of the fluids of an animal body by
          cathartics, venesection, or other means.
          [1913 Webster]

   2. That which is evacuated or discharged; especially, a
      discharge by stool or other natural means. --Quincy.
      [1913 Webster]

   3. Abolition; nullification. [Obs.] --Hooker.
      [1913 Webster]

   Evacuation day, the anniversary of the day on which the
      British army evacuated the city of New York, November 25,
      1783.
      [1913 Webster]

WordNet (r) 3.0 (2006):

evacuation
    n 1: the act of removing the contents of something [syn:
         emptying, voidance, evacuation]
    2: the act of evacuating; leaving a place in an orderly fashion;
       especially for protection
    3: the bodily process of discharging waste matter [syn:
       elimination, evacuation, excretion, excreting,
       voiding]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "evacuation":
   BM, abandonment, absentation, bloody flux, bowel movement,
   casting away, catharsis, cessation, cleaning out, clearance,
   clearing, crap, decampment, defecation, dejection, departure,
   depletion, desuetude, diarrhea, discharging cargo, disuse,
   drainage, draining, dysentery, egress, elimination, emptying,
   escape, excretion, exhausting, exhaustion, exit, exodus, flight,
   flux, forsaking, getaway, going, hegira, jettison, jettisoning,
   leaving, lientery, loose bowels, movement, off-loading, parting,
   passing, pulling out, purgation, purge, removal, retirement,
   retreat, runs, shit, shits, stool, throwing overboard, trots,
   turistas, unloading, venting, voidance, voiding, walkout,
   withdrawal