Gas in the Ethereum Virtual Machine (EVM) is a unit used to measure the computational work required to execute transactions and smart contracts on the Ethereum blockchain. Every operation performed by the EVM, such as transferring tokens, changing contract storage, or performing calculations, requires a certain amount of gas. More complex operations generally consume more gas than simpler ones.

Gas is necessary because Ethereum is a decentralised network where computers around the world process the same transactions and execute smart contracts. Without a mechanism that charges for computation, users could potentially submit unlimited or highly complex operations and consume network resources without sufficient cost. Gas therefore helps prevent spam, excessive computation, and certain types of denial-of-service attacks.

Gas should not be confused with the transaction fee itself. Gas represents the amount of computational work, while the fee is the amount paid for that work. The amount a user pays depends on the gas used and the applicable gas price or fee parameters. Ethereum commonly expresses gas prices in gwei, a small denomination of ETH.

Users typically specify a gas limit, which represents the maximum amount of gas they are willing to allow a transaction to consume. If execution requires more gas than the available limit, the transaction can fail with an out-of-gas error.

For developers, gas is an important consideration when designing smart contracts. Efficient code can reduce unnecessary computation and lower transaction costs. Overall, the EVM gas system helps allocate computational resources, discourage network abuse, and make blockchain execution economically manageable.