A digital ledger where data is recorded in a chronological order. In Bitcoin's case, it is a decentralized, public ledger which contains transactional information. Users can verify that transactions have occured simply by looking at the data that is publicized on the Bitcoin network.
In a blockchain, the next piece of information that will be added is always linked to a previous, already confirmed information of the blockchain through the use of a hash which describes the past content. With the hash, every block of information in a blockchain becomes referenced with one another, and cannot be easily swapped out, thereby qualifying as an immutable ledger.