Hallmarks are small impressions, struck in jewelry by official assay offices which serve as a type of consumer protection. Once a specific mark was impressed in the metal, the assay office would guarantee the fineness of the material used. The name comes from an English tradition where goldsmiths needed to offer their products for assay at Goldsmith's hall ("the hall") before they could sell them.
Only marks struck by official - government controlled - assay offices are to be named hallmarks. When the responsibility mark is struck by the maker, it is named a maker's mark.