The word niello, plural nielli, is the Italian form of the Latin word nigellum. Nigellum is a diminutive of niger, which means black. In the Middle Ages the term nigellum referred to the amalgam of black color used in 'nielloing'. In the Renaissance nigellum became niello; in addition to the amalgam it referred also to the objects to which the amalgam was applied.
Sir George Birdwood proposed the following definition, as one that would accurately
describe niello, providing at the same time a clear distinction from similar arts, for instance, damascening, enamelling, cloisonné, champlevé, bidri work, etc., "Niello is the process and the result of annealing (literally 'blackening," i.e., nielloing), or fixing by fusion, on a decoratively incised polished metal (usually silver, but occasionally gold) surface, an opaque, black (nonmercurial) amalgam of silver, copper and lead." 
In Thailand the official definition of nielloware is recorded in the Royal Dictionary, in its 1950 version the dictionary defines the word 'niello' as: "The manner of making receptacles by means of the application of a lead amalgam, superimposed upon or poured into incised designs. This type of container is referred to as 'nielloware'" 
In Thai the word used to refer to nielloware is khruang thom. According to the "Visitors Guide to the Nakhon Si Thammarat National Museum", the word thom is derived from thompa (Pali) or sathompa (Sanskrit). The meaning of thom is to fill up, depress, insert, affix and contain. Thus, khruang thom refers to the art of applying ya thom -- the niello liquid, to etched portions of a silver or gold objects in order to create patterns against a black background.