Gwei

Gas measures the computational cost of an operation: every opcode has an assigned cost. The gas price is expressed in gwei, and the total cost of a transaction is the gas used multiplied by the price per unit, paid in ETH and wei. After EIP-1559 (activated in the London hard fork, 5 August 2021), that price splits into a base fee set by the protocol and burned, and an optional priority fee that goes to the validator as a priority incentive. The user sets a max fee and a max priority fee instead of a single price, and the base fee adjusts automatically per block by up to plus or minus 12.5% depending on congestion.

Gwei is the usual reading unit: it lets fees be expressed manageably, since a fee of 0.000000020 ETH reads as "20 gwei". It is central to wallet user experience, to estimating the cost of deploying smart contracts, and to tokenization operations such as minting, transferring or claiming tokens. The wei, gwei and ether nomenclature comes from the Ethereum Yellow Paper (Gavin Wood); wei honors Wei Dai and gwei equals giga (10^9) wei.

MetaMask, Etherscan's Gas Tracker and Blocknative display fees in gwei. After EIP-1559, whose authors include Vitalik Buterin and Eric Conner, Ethereum entered a regime of base-fee burning. Layer 2 networks such as Arbitrum, Optimism, Base and zkSync batch transactions and settle on Layer 1, which notably reduces the gas the user pays; their fees are also expressed in gwei, but are orders of magnitude smaller.