Blockchain is a distributed ledger technology that uses cryptography to store data securely and immutably across a network of computers. It is a digital ledger of records, called blocks, which are linked together using cryptographic principles. Each block contains a timestamp and a link to the previous block, forming an immutable chain. This makes it virtually impossible to tamper with the data stored in the blockchain, as any changes would be easily detected by the network. In the context of JavaScript, blockchain can be used to create distributed applications (dApps) which are secure, transparent, and decentralized.