Blockchain
A shared record of transactions, copied across many computers, where new entries are grouped into blocks and linked in order.
Each block contains a cryptographic hash of the block before it, so altering an old entry would change every hash that follows and be immediately visible to anyone checking. Copies of the record are held by many independent operators, and a consensus rule decides which version of the chain everyone treats as canonical. The design exists so that parties who do not trust one another can share one ledger without appointing an operator to maintain it. A reader meets the term as the base layer of networks such as Bitcoin and Ethereum, and in enterprise settings where the same structure is run by a closed group.
In practice
Bitcoin's chain begins with a genesis block created in January 2009, and every block since references the hash of the block before it.
The common misunderstanding
A blockchain does not make the information written into it true; it makes the recorded order and history hard to alter, not the claims themselves accurate.