Ethereum is a platform and a programming language that makes it possible to build and publish next-generation distributed applications using crytocurrency tech like decentralized blockchains.
From Wikipedia: Ethereum is a decentralized Web 3.0 publishing platform featuring stateful user-created digital contracts and a Turing-complete contract programming language. Ethereum uses its underlying network unit, ether, as payment to execute Ethereum contracts as a workaround to the Halting Problem. In this respect, Ethereum is unlike most cryptocurrencies, as it is not solely a network for transacting monetary value, rather, it is a network for powering Ethereum-based contracts.
- self-executing contracts so that trusted thrd parties and centralization are unnecessary