Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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