Ledger
A ledger is the record of all transactions; on a blockchain it is public, append-only and shared by every node.
In depth
Banks keep private ledgers you cannot inspect. A blockchain ledger is the opposite — anyone can download it and audit every movement since the genesis block. Balances are not stored as numbers in an account; they are computed from the full history of transactions.
Why it matters
Transparency of the ledger is what allows independent verification and auditability.