Gas is the transaction fee paid by users interacting with the Ethereum Blockchain. The amount of gas paid in a transaction is calculated based on the computational complexity of the executed smart contract code. Specifically, there is a gas cost for each EVM opcode. For example, storing data in a smart contract using the "SSTORE" opcode costs 20,000 gas. Gas is a visual representation of a gas meter composed of every EVM opcode mapped to its associated gas cost.
