Glossary
Gwei
A denomination of ether equal to one billionth (10⁻⁹) of one ETH, used almost universally to quote gas prices because raw wei figures are unwieldy.
Ether is divisible down to the wei, the smallest unit, where one ETH equals 10¹⁸ wei. Working in wei is awkward for everyday fees, so Ethereum tooling expresses gas prices in gwei, short for "giga-wei" — one gwei is 10⁹ wei, or one billionth of an ETH.
When a wallet shows a base fee of 15 gwei or a tip of 2 gwei, it is describing the price paid per unit of gas. Multiplying the gas price in gwei by the gas a transaction consumes, then converting back to ETH, gives the total fee. The unit keeps numbers readable: a few gwei is a normal price, while tens or hundreds signal a congested network.