SAP, in sieges, is a trench, or an approach made under cover, of 10 or 12 feet broad, when the besiegers come near the place, and the fire from the garrison grows so dangerous that they are not able to approach uncovered.—There are several sorts of saps; the single, which has only a single parapet; the double, having one on each side; and the flying, made with gabions, &c. In all saps traverses are left to cover the men.