Greek δίδραχμον. Literally "two drachmas". Also called Two drachmas, Temple tax, didrachmon.
Two drachmas — the annual temple tax, and the only tax the gospels show being collected.
Wage
2 days' wages (2 days' wages).
Reckoned from two drachmas at a day's wage each.
Diameter
1.9–2.1 cm (0.748–0.827 in).
Reckoned from surviving half-shekels of Tyre, the coin the temple tax was paid in.
The didrachma is a two-drachma coin, and it is in the New Testament for one reason: it is the half-shekel every Jewish man owed the temple each year, the same head tax Exodus sets at a beka. The collectors ask Peter whether his teacher pays it, which is a question about whether the temple has a claim on him at all.
Sizes are the range of surviving examples, and coins of one name varied from mint to mint. Bronze paid for small things and silver for wages and taxes: the widow gave two lepta (Mark 12:42), the temple tax was a didrachma (Matthew 17:24), and a stater paid it for two (17:27).