From 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]