BIVOUAC (a French word generally said to have been introduced during the Thirty Years' War, perhaps derived from Beiwacht, extra guard), originally, a night-watch by a whole army under arms to prevent surprise. In modern military parlance the word is used to mean a temporary encampment in the open field without tents, as opposed to "billets" or "cantonment" on the one hand and "camp" on the other. The use of bivouacs permits an army to remain closely concentrated for all emergencies, and avoids the necessity for numerous wagons carrying tents. Constant bivouacs, however, are trying to the health of men and horses, and this method of quartering is never employed except when the military situation demands concentration and readiness. Thus the outposts would often have to bivouac while the main body of the army lay in billets.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)