in Fortification, a work of earth, sometimes in the form of a simple parapet, and at others like a small rampart with a parapet. It is sometimes raised on the ditch, and sometimes beyond it. Envelopes are of a zigzag form, in order to enclose a weak ground, with single lines, where that is practicable, or to save the expense of horn-works, crown-works, and tennailles, where room is wanting for such defences. Envelopes in a ditch are called sillons, counter-guards, and lunettes.