A two-dimesional pattern that can be folded to make a three-dimesional figure.
Informal: If you cut the surface of a polyhedron (a three-dimensional geometric figure bounded entirely by polygons) along its edges and unfold it flat, you get the figure's net. The polygons that comprise the net correspond to the faces of the polyhedron.
Because there is more than one way of cutting edges and unfolding, all polyhedra have more than one net. [Need example here.]