A ledger is a record of financial transactions. In crypto, it's a record of all transactions on a blockchain.
In accounting, a ledger is a book containing accounts in which the classified and summarized information from the journals is posted as debits and credits. It provides a record of all financial transactions over the life of the company. In cryptocurrencies, a ledger is a record of all transactions made within a certain cryptocurrency network. The Bitcoin Ledger, for example, is a record of all transactions made with Bitcoin. This ledger is stored on all computers running the Bitcoin software and is updated regularly as new transactions are added to the blockchain.
An example of a ledger in crypto would be the Bitcoin blockchain, where every single Bitcoin transaction since the beginning is recorded and publicly available for review.