Envelope

In fortification, a work of earth, sometimes in the form of a single parapet, and at others like a small rampart; it is raised sometimes in the ditch, and sometimes beyond it. Envelopes are occasionally en zigzag , to inclose a weak ground, where that is practicable, with single lines. Envelopes, in a ditch, are sometimes called sillons, contregardes, conserves, lunettes, etc.