Shared, trusted, public ledger of transactions, that everyone can inspect but which no single user controls. It is a cryptographed, secure, tamper-resistant distributed database. It solves a complex mathematical problem to exist. A blockchain is a perfect place to store value, identities, agreements, property rights, credentials, etc. Once you put something like a Bitcoin into it, it will stay there forever. It is decentralized, disintermediated, cheap, and censorship-resistant. Applications of Blockchain: Bitcoin (cryptocurrency), Namecoin (wants to replace the entire DNS system of the Internet), or Sia (decentralized cloud storage), Ethereum (Turing complete Virtual Machine where you can run any smart contract); Any centralized service like eBay, Dropbox can potentially be built in a decentralized way using blockchain technology, considerably lowering transaction costs.