or ABATTIS, is, in military language, the name of a kind of retrenchment made of felled trees. When the emergency is sudden, the trees are merely laid lengthwise beside each other, with their branches pointed towards the enemy, to prevent his approach, whilst the trunks serve as a breastwork before those by whom the abatis is raised. When the abatis is meant for the defence of a pass or entrance, the boughs of the trees are generally stripped of their leaves and pointed; the trunks are planted in the ground; and the boughs are interwoven with each other. It is needless to add, that the closer the trees are laid or planted together, the more secure is the defence which they afford; and if, when they are planted, a small ditch be dug towards the enemy, and the earth thrown up properly against the lower part of the abatis, it will be very difficult to pass it if well defended.—Simler's Military Guide.

ABBREVIATION or FRACTIONS, in arithmetic and algebra, is the reducing of them to lower terms; which is done by dividing the numerator and denominator by some number or quantity which will divide both without leaving a remainder of either.