Gwei
The unit gas fees are priced in on Ethereum — one billionth of an ETH.
Gwei is a denomination of ETH used to price gas fees: one gwei is 0.000000001 ETH (10⁻⁹). Because a single transaction’s fee is a tiny fraction of an ETH, quoting it in gwei keeps the numbers human-readable — “20 gwei” is far easier than writing out the ETH decimal.
When you see network congestion discussed as “gas is at 80 gwei,” that’s the per-unit price of computation on Ethereum, which multiplies by the transaction’s complexity (its gas limit) to produce the total fee. Watching gwei is how Ethereum users time transactions — waiting for quiet periods when it drops can cut a transaction’s cost several-fold.