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What is a blockchain?
A blockchain is a shared record of transactions, copied across many computers that check each new entry against the same rules. Entries are grouped into blocks, and each block carries a fingerprint (hash) of the one before it, so altering an old entry would break every block after it.
What it means for you
Once your transaction is recorded on the chain, no bank or support desk can reverse it; only the recipient can send the funds back. A blockchain records who received what, not whether the payment was a good idea or the recipient is who they claim to be.
How it works
Each block records a batch of transactions plus a header that includes a hash of the previous block, so every block references its parent. Nodes each keep a copy and check every new block against the protocol rules, rejecting any that break them. Because each block commits to the one before, editing an old transaction changes that block's hash and breaks the link to every later block. On Bitcoin, an attacker would have to redo the proof of work for that block and all blocks after it, then catch up with and overtake the honest chain, a cost that grows with every block added.
Sources: ethereum.org: Intro to Ethereum, bitcoin.org: Frequently Asked Questions, Bitcoin developer guide: Block chain, Bitcoin whitepaper (PDF), Satoshi Nakamoto · checked 4 October 2026
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