Blockchain is the technology that makes cryptocurrency possible. It is a network of distributed ledgers, called “blocks.” Transactions are logged into these ledgers. The entries in their turn are immutable — they cannot be changed or erased once they have been entered.
It helps to think of a blockchain as a spreadsheet that is copied on thousands of computers on a network. Then imagine the network updating this spreadsheet regularly on each of the computers. That’s what a blockchain is, in essence.
Other interesting terms…