Definition of nielli:
(n) :
(metallurgy) Any of various black metal alloys, made of sulphur with copper, silver or lead, used to create decorative designs on other metals.
(v) :
(transitive) To decorate (a surface) using this technique.
(n) :
An impression on paper taken from the engraved or incised surface before the niello alloy has been inlaid.