The Collaborative International Dictionary of English v.0.48:

Fort \Fort\, n. [F., from fort strong, L. fortis; perh. akin to
   Skr. darh to fix, make firm, and to E. firm Cf. Forte,
   Force, Fortalice, Comfort, Effort.] (Mil.)
   A strong or fortified place; usually, a small fortified
   place, occupied only by troops, surrounded with a ditch,
   rampart, and parapet, or with palisades, stockades, or other
   means of defense; a fortification.
   [1913 Webster]

         Detached works, depending solely on their own strength,
         belong to the class of works termed forts. --Farrow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fort
    n 1: a fortified military post where troops are stationed [syn:
         garrison, fort]
    2: a fortified defensive structure [syn: fortress, fort]
    v 1: gather in, or as if in, a fort, as for protection or
         defense [syn: fort, fort up]
    2: enclose by or as if by a fortification [syn: fortify,
       fort]
    3: station (troops) in a fort

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "fort":
   acropolis, bastion, beachhead, blockhouse, bridgehead, bunker,
   castle, citadel, donjon, fasthold, fastness, fortress, garrison,
   garrison house, hold, keep, martello, martello tower, mote, motte,
   peel, peel tower, pillbox, post, rath, redoubt, safehold,
   strong point, stronghold, tower, tower of strength, ward