Wei is the smallest unit of ether—the cryptocurrency coin used on the Ethereum network. One ether = 1,000,000,000,000,000,000 wei (1018). The other way to look at it is one wei is one quintillionth of an ether.

The wei is named after Wei Dai, best known as the creator of the bitcoin predecessor "b-money" and as the developer of the Crypto++ library.