An Ethereum smart contract is a self-executing contract that runs on the Ethereum blockchain. It contains rules and conditions written in code, which automatically execute when predefined conditions are met. Ethereum smart contracts enable decentralized applications (dApps) to operate without intermediaries, offering trustless, transparent, and secure transactions. They are typically written in Solidity, Ethereum's programming language, and are stored and executed on the Ethereum network. These contracts are widely used in areas like finance (DeFi), gaming, and NFTs.