What is an invoice?

The dictionary definition of an invoice is "a list of goods sent or services provided, with a statement of the sum due for these; a bill."

An invoice is a commercial document issued by a seller to a buyer. The payment terms are normally stated on the invoice. This means it would say that the buyer has a certain number of days in which to pay and could receive a discount if they pay before the due date. Another example could be that the buyer could have already paid for the product of services stated on the invoice.

For a seller, an invoice is a sales invoice.
For a buyer, an invoice is referred to as a purchase invoice. 

The term invoice indicates money is owed or owing, the document itself indicates the buyer and the seller.

A typical invoice includes:

• The word invoice
• A unique reference number
• Date of the invoice
• Credit terms
• Tax payments (not always relevant)
• Name/contact details of the seller
• Name/contact details of the buyer
• Tax or company registration details of seller 
• Date the goods or service was sent/delivered
• Purchase order number 
• Description of the product
• Unit price of the product
• Total amount charged
• Payment terms (this included the method of payment, date of payment, details about charges for late payment)