Narrow sense
Blockchain is a kind of distributed ledger technology in which data blocks are sequentially linked into a chained data structure in a chronological order and are cryptographically guaranteed to be tamper-proof and unforgeable.
Broad sense
Blockchain technology is a new distributed infrastructure and computing paradigm that uses block-chain data structure to verify and store data, distributed node consensus algorithms to generate and update data, cryptography to secure data transmission and access, and smart contracts composed of automated scripting code to program and manipulate data.
In short, blockchain is a decentralized distributed ledger database. Cryptography is used to ensure that existing data cannot be tampered with, so the data or information stored in it is decentralized and untamperable.
These features ensure the "honesty" and "transparency" of the blockchain and become blockchain's rock of the trust. These features enable blockchain to solve many asymmetrical information problems, allowing users to collaborate without the participation of third-party intermediaries and without having to trust each other, realizing collaborative trust and consistent action among multiple subjects, thus creating a reliable cooperation mechanism with broad and rich application prospects.