The smallest unit of Ether, Ethereum’s native cryptocurrency, is called a wei.
One ether is divisible into 10^18 wei. The unit is named after Wei Dai, a computer scientist associated with the development of the b-money concept and the cypherpunk movement. Ethereum uses wei for precise accounting because transaction fees, contract payments, and token calculations can involve extremely small fractions of an ether.
Users more commonly encounter gwei, which equals one billion wei. Ethereum transaction fees are usually quoted in gwei because displaying them directly in wei would be less readable. Ether itself is the currency, while wei is its smallest denomination; this parallels bitcoin’s relationship with the satoshi, although the number of subdivisions differs.
Ethereum applications and smart contracts generally record balances as integer amounts of wei. Wallet interfaces then convert those values into ether or gwei for human-readable display.