A prepayment is money you have already paid for something you have not received yet — a deposit on a booking, goods ordered ahead, rent paid for a month still to come.

The money has left your account but you are not out of pocket: the seller owes you delivery, so the amount sits on the "own" side of your net worth until it arrives. Recording the delivery is what finally turns it into spending.

It is the mirror image of a credit purchase: there you have the goods and owe the money, here you have paid the money and are owed the goods.