[en.ve.lope] n (ca. 1714) 1: a flat usu. paper container (as for a letter)
2: something that envelops: wrapper "the ~ of air around the earth" 3 a: the outer covering of an aerostat b: the bag containing the gas in a balloon or airship
4: a natural enclosing covering (as a membrane, shell, or integument)
5. a: a curve tangent to each of a family of curves b: a surface tangent to each of a family of surfaces
6: a set of performance limits (as of an aircraft) that may not be safely exceeded; also: the set of operating parameters that exists within these limits