(Brit) an official series of marks, instituted by statute in 1300, and subsequently modified, stamped by the Guild of Goldsmiths at one of its assay offices on gold, silver, or platinum (since 1975) articles to guarantee purity, date of manufacture, etc
a mark or sign of authenticity or excellence
an outstanding or distinguishing feature
(transitive) to stamp with or as if with a hallmark
Also (for senses 1, 4) platemark
C18: named after Goldsmiths' Hall in London, where items were graded and stamped
1721, official stamp of purity in gold and silver articles, from Goldsmiths' Hall in London, site of the assay office; see hall + mark (n.1). General sense of "mark of quality" first recorded 1864. As a verb from 1773.