The Collaborative International Dictionary of English v.0.48:

Abatis \Ab"a*tis\, Abattis \Aba"t*tis\, ([a^]b"[.a]*t[i^]s;
   French [.a]`b[.a]`t[=e]") n. [F. abatis, abattis, mass of
   things beaten or cut down, fr. abattre. See Abate.] (Fort.)
   A means of defense formed by felled trees, the ends of whose
   branches are sharpened and directed outwards, or against the
   enemy.
   [1913 Webster]

WordNet (r) 3.0 (2006):

abatis
    n 1: a line of defense consisting of a barrier of felled or live
         trees with branches (sharpened or with barbed wire
         entwined) pointed toward the enemy [syn: abattis,
         abatis]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "abatis":
   advanced work, balistraria, bank, banquette,
   barbed-wire entanglement, barbican, barricade, barrier, bartizan,
   bastion, battlement, breastwork, bulwark, casemate,
   cheval-de-frise, circumvallation, contravallation, counterscarp,
   curtain, demibastion, dike, drawbridge, earthwork, enclosure,
   entanglement, escarp, escarpment, fence, fieldwork, fortalice,
   fortification, glacis, loophole, lunette, machicolation, mantelet,
   merlon, mound, outwork, palisade, parados, parapet, portcullis,
   postern gate, rampart, ravelin, redan, redoubt, sally port, scarp,
   sconce, stockade, tenaille, vallation, vallum, work



The Devil's Dictionary (1881-1906):

ABATIS, n.  Rubbish in front of a fort, to prevent the rubbish outside
from molesting the rubbish inside.