Gas Limit

A gas limit is the maximum amount of computational work a sender authorises a transaction to consume. Gas measures execution effort rather than clock time, so a plain transfer needs very little of it while a complex smart contract call needs far more. If execution finishes below the limit, the unused portion is not charged. If execution hits the limit first, the transaction fails, any state changes are reverted, and the gas already spent is still paid. Blocks carry their own gas limit too, capping total work per block. Wallets estimate the value automatically, but estimates can be wrong when contract state changes between estimation and execution, which is a common reason transactions fail with an out of gas error.