A blockchain is a decentralized and distributed digital database that records transactions on a network of computers. Essentially, it is a kind of public digital ledger where transactions or data blocks are securely, transparently, and immutably recorded.

The data in a blockchain is stored in blocks that are secured by complex cryptographic algorithms, which make it extremely difficult to modify or delete a block once it has been stored in the blockchain. Each block contains a cryptographic hash of the previous block, which means that each block refers to the previous block and forms a chain of blocks – hence the name blockchain.

A blockchain can be used for various purposes, such as for cryptocurrencies like Bitcoin, for smart contracts, for the transfer of data, or for securing valuables. The technology offers a secure and transparent way to store and transfer information or value without relying on a central authority or a trusted third party.