A blockchain is a ledger of transactions that grows in real time and that is validated at the end of each block (or transaction cycle), so that each existing block as well as the entire chain of blocks to date is validated (in the form of a Merkle tree). Validation takes the form of a hash function applied to blocks individually as well as across blocks, and thus ensures that the ledger formulated as a blockchain has not been tampered with (such as someone attempting fraudulently to modify bookkeeping entries for past transactions).
Blockchain is a central concept of cryptocurrencies. The anonymous Bitcoin founder Satoshi Nakamoto, introduced the concept in his original white paper laying out Bitcoin. Even though the actual term “blockchain” appears nowhere in that whitepaper, the term “block” appears often in it and the terms “ongoing chain” and “record that cannot be changed” appear in the abstract. These all refer to the same idea, namely, a cryptocurrency blockchain, or simply blockchain.
Cryptocurrency blockchains constitute a complete record of all transactions ever conducted in the underlying cryptocurrency. For instance, to see every Bitcoin transaction ever conducted, go to the “explorer” feature at Blockchain.com.