Blockchain.
A definition, in plain English — with the books that teach it.
What it means
A shared, append-only ledger that thousands of computers keep identical copies of. Transactions are grouped into 'blocks' and cryptographically chained to the prior block, so rewriting history would mean redoing every block since — which is computationally impractical on a large network. The honest caveat: 'blockchain' alone isn't a feature — what matters is which blockchain, who validates it, and what guarantees that actually buys you.
Example
When you send Bitcoin, your transaction sits in a 'mempool' until a miner includes it in the next block (~10 minutes). Once it's six blocks deep, the network treats it as final — undoing it would require outspending the rest of the miners combined.
