Golden drachma

A Golden Drachma is Greek currency. They are large round coins that can be made up of pure gold or silver but it is stated in The Lightning Thief, that the gods never use anything less than pure gold. They are described as big as Girl Scout cookies by Percy and are used by the gods and demigods.

Uses
The drachmas have images of various gods stamped on one side and the Empire State Building on the other. The ancient mortal drachmas were made of silver, but the gods now do not use anything less than pure gold mined from Olympus. Golden drachmas are used to send Iris messages.

Percy Jackson and the Olympians
A drachma can be used to create an Iris Message; as payment to get into the Underworld; to pay the Gray Sisters to take you anywhere as long as its within their serving area; or as payment for sending a letter or package along the Hermes Express. Currently, the pure gold drachma is the main currency of the gods, goddesses, and demigods.

The most common use for the drachma is to send an Iris message through the rainbow goddess. It is used for contact when nothing else is available, and is quite useful if any extra drachmas are allowed for use.

Percy also uses some drachmas to bribe Charon when he first enters the Underworld.

The Heroes of Olympus
Roman demigods don't use drachma's because it's Greek currency. Instead they use denarii. The Romans never used Iris Messages like the Greeks because they relied more on their giant eagles and road systems.

Video Game
In the Percy Jackson and the Lightning Thief Video Game, drachmas aren’t used as currency, instead they are used as equipment for characters. They increase certain attributes to whoever equips it. Each character can equip two. There are 20 drachmas that the player can collect in the game.

Trivia

 * Drachmas are Greek currency, while Aureus or denarii are Roman currency.
 * The word drachma came from the word drássomai, which means "to grasp."