Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

   3. A piece of metal, or any other object, so decorated.

   4. An impression on paper taken from an ancient incised
      decoration or metal plate.

Niello \Ni*el"lo\, n.
   An impression on paper taken from the engraved or incised
   surface before the niello alloy has been inlaid.