an official mark or stamp indicating a standard of purity, used in marking gold and silver articles assayed by the Goldsmiths' Company of London; plate mark.

any mark or special indication of genuineness, good quality, etc.

any distinguishing feature or characteristic

Accuracy is a hallmark of good scholarship.

to stamp or imprint (something) with a hallmark.

noun

badge , certification , device , emblem , endorsement , indication , mark , ratification , seal , sign , signet , stamp , sure sign , telltale sign , trademark , characteristic , symbol