1 authentication, verification, seal or stamp (of authenticity or approval), mark, device, sign, symbol; plate-mark; assay-mark:
From this hallmark we know when and where the piece was made, by whom, and its degree of purity. Surprise endings are the hallmark of O. Henry's short stories.
2 feature, stamp, mark, earmark, trade mark, characteristic, identification:
From his hallmark, a tiny axe, you can tell that the painting is a genuine Fellworthy.