Answer
Books of prime entry (also called books of original entry or day books) are where transactions are first recorded from source documents before being posted to the ledgers. They include the sales day book, purchases day book, sales and purchases returns day books, the cash book, the petty cash book, and the journal. They group similar transactions to make posting to the ledger efficient.
Real-world example A bookkeeper lists each credit sale invoice in the sales day book during the day, then posts the totals to the ledger later.