WordNet (r) 3.0 (2006):

concentration camp
    n 1: a penal camp where political prisoners or prisoners of war
         are confined (usually under harsh conditions) [syn:
         concentration camp, stockade]
    2: a situation characterized by crowding and extremely harsh
       conditions

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "concentration camp":
   Belsen, Konzentrationslager, Lager, POW camp, abattoir, aceldama,
   barrack, barracks, bastille, bivouac, black hole, borstal,
   borstal institution, bridewell, brig, butchery, camp, campground,
   campsite, cantonment, casern, cell, condemned cell, death cell,
   death house, death row, detention camp, encampment, federal prison,
   field of blood, forced-labor camp, gaol, gas chamber, guardhouse,
   hobo jungle, house of correction, house of detention,
   industrial school, internment camp, jail, jailhouse, keep,
   labor camp, lines, lockup, maximum-security prison,
   minimum-security prison, oubliette, pen, penal colony,
   penal institution, penal settlement, penitentiary, prison,
   prison camp, prisonhouse, reform school, reformatory, shambles,
   slaughterhouse, sponging house, state prison, stockade, stockyard,
   the hole, tollbooth, training school