Wei is the smallest denomination of Ether, the native crypto of the Ethereum network. One Ether is equivalent to 1,000,000,000,000,000,000,000 wei or commonly written as 1e18 wei. This smallest unit is commonly used to make transactions and is used when referring to the price of transaction fees or gas fees.
The term Wei is named after cryptography activist Wei Dai who is known for facilitating the widespread use of cryptography. Wei Dai created Bitcoin’s predecessor, B-Money.