Gas

aka gasoline, fossil fuel for automobiles. Also ethereum transaction cost

Gas is a unit of account within the EVM used in the calculation of the transaction fee, which is the amount of ETH a transaction's sender must pay to the network to have the transaction included in the blockchain. Each type of operation which may be performed by the EVM is hardcoded with a certain gas cost, which is intended to be roughly proportional to the monetary value of the resources (e.g. computation and storage) a node must expend or dedicate to perform that operation. https://en.wikipedia.org/wiki/Ethereum#Gas