The Collaborative International Dictionary of English v.0.48:

Niello \Ni*el"lo\, n. [It. niello, LL. nigellum a black of
   blackish enamel, fr. L. nigellus, dim. of niger black. See
   Negro, and cf. Anneal.]
   1. A metallic alloy of a deep black color.
      [1913 Webster]

   2. The art, process, or method of decorating metal with
      incised designs filled with the black alloy.
      [1913 Webster]

   3. A piece of metal, or any other object, so decorated.
      [1913 Webster]

   4. An impression on paper taken from an ancient incised
      decoration or metal plate.
      [1913 Webster]

   5. An impression on paper taken from the engraved or incised
      surface before the niello alloy has been inlaid.
      [Webster 1913 Suppl.]