A blockchain database is a decentralised, distributed digital ledger that records transactions across multiple computers in a secure, transparent, and immutable way. Unlike traditional databases managed by a central authority, a blockchain operates on a peer-to-peer network where each participant (or node) maintains a copy of the entire ledger.

Each record, called a "block," contains a list of transactions, a timestamp, and a cryptographic hash of the previous block, creating a secure, chronological chain. This structure ensures that once data is recorded, it cannot be altered without consensus from the network, making blockchain highly resistant to fraud and tampering.

Blockchain databases use consensus mechanisms like Proof of Work (PoW) or Proof of Stake (PoS) to validate transactions. These mechanisms ensure all nodes agree on the state of the database, maintaining consistency across the network.

Blockchains can be public (e.g., Bitcoin, Ethereum), where anyone can participate, or private/permissioned, restricted to specific users or organisations. They’re used beyond cryptocurrencies, including in supply chain management, healthcare, finance, and digital identity.

The key advantages of blockchain databases include transparency, security, decentralisation, and auditability. However, they may face challenges like scalability, energy consumption, and regulatory acceptance depending on the use case and design.