The Collaborative International Dictionary of English v.0.48:

Garrison \Gar"ri*son\, v. t. [imp. & p. p. Garrisoned; p. pr.
   & vb. n. Garrisoning.] (Mil.)
   (a) To place troops in, as a fortification, for its defense;
       to furnish with soldiers; as, to garrison a fort or town.
   (b) To secure or defend by fortresses manned with troops; as,
       to garrison a conquered territory.
       [1913 Webster]