Blockchain
A shared, append-only ledger that a network of computers agrees on without a central operator. Nobody can quietly edit the history.
A blockchain is a database with two unusual properties. First, it is append-only: you can add records but you cannot rewrite the ones already there without every participant noticing. Second, no single party controls it. A network of nodes runs the same software and agrees on the next block through a consensus mechanism (proof-of-work, proof-of-stake, or a permissioned variant). The result is a ledger multiple parties can trust without trusting each other.
There are two broad kinds. A public blockchain (Ethereum, Bitcoin, Solana) is open: anyone can read it, run a node, or transact. A private or permissioned blockchain restricts who participates, which usually means you have rebuilt a slower, more complex version of a normal database with extra steps. If one company controls all the nodes, you do not have a blockchain advantage, you have a distributed system you now have to operate.
The honest stance: most projects pitched as needing a blockchain do not. A blockchain earns its complexity only when you need assets that survive the operator’s bankruptcy, shared state between parties who do not trust each other, or permissionless composability. Everything else is a database. We have shipped real blockchain systems and we have talked clients out of more than we built. Our blockchain work starts with whether you need one at all.