Hebrew שֶׁקֶל. Literally "a weighing". Also called Sheqel, Sicle.
The standard weight of silver, and the anchor every other weight is a multiple or fraction of.
Mass
11–11.5 g (0.388–0.406 oz).
Reckoned from inscribed stone weights excavated across Iron Age Judah.
For most of the Old Testament a shekel is not a coin but a weight — the word means "a weighing", and silver was weighed out on scales until coinage reached Judah in the Persian period. Inscribed stone shekel weights have been excavated across Judah in numbers, and they cluster tightly around 11.4 g, which makes this the best-attested measure on this page. The sanctuary shekel is named as a standard, which implies an ordinary one that could drift.
Silver was weighed, not counted, until coins reached Judah, so each weight is drawn as the cube that much silver makes. A shekel is about a centimetre across; the thirty pieces of silver would sit in a palm; a talent is a block a hand and a half wide.
Where it appears
Exodus 30:13The shekel of the sanctuary, twenty gerahs
Genesis 23:16Abraham weighs out four hundred shekels for a burial plot