I’ll paste an example definition:
Blockchain is a system of recording information in a way that makes it difficult or impossible to change, hack, or cheat the system . A blockchain is essentially a digital ledger of transactions that is duplicated and distributed across the entire network of computer systems on the blockchain.
Bitcoin is a ledger. Like if you had a sheet of paper and you gave your son 1 coin by recording his name, then a friend 2 coins, then your friend paid you for babysitting 0.5 coins, etc., etc., and the ledger kept track of who did what and the balance could always be calculated WITHOUT ERROR OR THREAT OF AN ERASER/CHANGER from the transactions that occurred before on the ledger.
In the above, on the bitcoin blockchain, we know that address ** bc1qu4q6jgutzw9adh238h3kckrxkyv720we30cwng** received 2.50853715 BTC with a current exchange rate of $109,855.86. We do not know WHO owns the keys to that address on the blockchain, but even within as short as an hour or so, this transaction will be SO SECURE through encryption that it will be considered “immutable” – meaning there’s no way to change it.
When it comes to money – what’s yours and what is everyone else’s – this is a huge breakthrough! But it is not “blockchain” that is the core breakthrough. It is the combination of technologies that came together to make this ledger work the way it does: public, decentralized, censorship-resistant, permissionless, immutable (non-changeable), as well as ownership being transferable anywhere within hours (or seconds on the lightning network) – whether $5 or $5 billion – irreversibly. Once it is recorded on the bitcoin blockchain, the person(s) with the private keys are the only ones who control those sats.