Ethereum is a decentralized, open-source blockchain platform that runs smart contracts: applications that run exactly as programmed without any possibility of downtime, censorship, fraud, or third-party interference.
These applications are run on a custom-built blockchain, an enormously powerful shared global infrastructure that can move value around and represent the ownership of property.
In addition, Ethereum is the most widely used blockchain platform in the world, and is used by thousands of developers and companies to build and deploy decentralized applications.