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):

abattis
    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]