What is the blockchain?
Bitcoin's shared record of every payment since 2009, stored in blocks that each link to the one before — which is what makes it a chain.
Each block carries a batch of transactions and the fingerprint of the block before it. Change any past transaction and every fingerprint after it breaks, so the history can't be quietly edited: nodes would reject the altered chain.
Every node keeps its own copy and checks it against the same rules, which is why no one has to trust a central record-keeper. “Blockchain” has become a buzzword for other uses, but on this site it means Bitcoin's chain.