Distinctive characteristics or features that identify something as authentic or typical; originally, official marks stamped on precious metals to guarantee their quality.
From 'hall mark,' a compound of 'hall' (the Goldsmiths' Hall in London where assaying was done) and 'mark.' Originally a literal stamp, it became metaphorical by the 1800s to mean any distinguishing characteristic.
The literal hallmark originated in medieval London when the Goldsmiths' Hall needed to stamp gold and silver to prove it wasn't fake—so now any distinctive mark that proves authenticity borrows this word! It's a great example of how a specific practice becomes a universal metaphor.
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