The Collaborative International Dictionary of English v.0.48:

Nappe \Nappe\, n. [F. nappe cloth, sheet. See Napery.] (Geom.)
   Sheet; surface; all that portion of a surface that is
   continuous in such a way that it is possible to pass from any
   one point of the portion to any other point of the portion
   without leaving the surface. Thus, some hyperboloids have one
   nappe, and some have two.
   [1913 Webster]