A ‘wei’ refers to the smallest denomination of ether (ETH).
Understanding the Term
One wei (pronounced: “way”) is the smallest fraction of ether. Ether is a cryptocurrency token upon which the Ethereum network is based. That is to say:
1 wei = one quintillionth of ether (ETH)
1 ETH = 1×1018 wei
A wei is to ether as a satoshi is to Bitcoin — both units are the smallest unit from which a user may make a transaction.