A form of bookkeeping, devised centuries ago, whereby each transaction is entered twice, a transaction being by its nature two-sided. The purchase of an asset necessitates the recording of the asset and also, separately, of the debt incurred or the money paid. The book in which all the accounts are kept is known as the ledger, though the cash account is normally kept in a separate book. known as the cash book. At any time the total debit balances in the ledger should equal the total credit balances. If this is so, it is an indication that (in the absence of compensating errors) double entries have been properly made. This is because, of the two entries made, one is always a debit and one a credit entry.
Reference: The Penguin Business Dictionary, 3rd edt.