The Collaborative International Dictionary of English v.0.48:

Encampment \En*camp"ment\, n.
   1. The act of pitching tents or forming huts, as by an army
      or traveling company, for temporary lodging or rest.
      [1913 Webster]

   2. The place where an army or a company is encamped; a camp;
      tents pitched or huts erected for temporary lodgings.
      [1913 Webster]

            A square of about seven hundred yards was sufficient
            for the encampment of twenty thousand Romans.
                                                  --Gibbon.
      [1913 Webster]

            A green encampment yonder meets the eye. --Guardian.
      [1913 Webster]

WordNet (r) 3.0 (2006):

encampment
    n 1: a site where people on holiday can pitch a tent [syn:
         campsite, campground, camping site, camping ground,
         bivouac, encampment, camping area]
    2: temporary living quarters specially built by the army for
       soldiers; "wherever he went in the camp the men were
       grumbling" [syn: camp, encampment, cantonment,
       bivouac]
    3: the act of encamping and living in tents in a camp [syn:
       camping, encampment, bivouacking, tenting]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "encampment":
   Konzentrationslager, Lager, barrack, barracks, bivouac,
   bivouacking, camp, campground, camping, campsite, cantonment,
   casern, concentration camp, detention camp, hobo jungle, lines,
   tented field, tenting