A fee implemented for each operation performed on a certain blockchain such as Ethereum. If a blockchain requires payment in the form of Gas, the network is unlikely to be stuck having to do useless work. The gas cost to a user depends on the intensity of use.
The highest amount of gas one is willing to pay in transaction fees. A transaction fails if the gas limit is not met, miners end transaction processing and keep the gas as payment for processing power despite its verification failure. Gas fees are designed to reduce the network's burden that is caused by useless work.
A person initiating a transaction on certain blockchains, such as Ethereum's, has to pay a fee to the miners for it to be appropriately validated. This fee is called the gas price and paid in ether. The gas price is dependent upon the work required to execute a smart contract, and the initiator of the transaction may pay more gas to prioritize the transaction.