a black metallic substance, consisting of silver, copper, lead, and sulfur, with which an incised design or ground is filled to produce an ornamental effect on metal.
2.
ornamental work so produced.
3.
a specimen of such work.
4.
a print on paper made from an incised metal plate before the application of niello and associated esp. with 15th-century Italy.
–verb (used with object)
5.
to decorate by means of niello; treat with niello or by the niello process.
Origin: 1810–20; < It < L nigellus blackish, dim. of niger black
[nee-el-oh] 
noun, plural -el⋅li 